Telecommunications system

ABSTRACT

A communication procedure suitable for a cordless telephone system involves time division duplex radio communication between a handset 11 and a base station 3 using alternating bursts of transmission over a single radio channel. Once a radio link has been set up, initial transmissions carry a synchronisation logical channel S and a signalling logical channel D multiplexed together, but the link may switch to bursts carrying a communications logical channel B for the speech data and the signalling logical channel D. Burst synchronisation is achieved by the asynchronous detection of words in a synchronisation channel S. These words have bit patterns reducing the probability of incorrect asynchronous detection of them.   If one part ceases to receive handset signals from the other, it transmits a special signal, informing the other part. This enables both parts to detect the failure of a link at substantially the same time, so that their actions to re-establish the link are synchronised.

FIELD OF THE INVENTION

The present invention relates to telecommunications systems, and hasparticular application to cordless telephones. Aspects of the inventionare useful in so called "CT2" cordless telephone systems, and systems inaccordance with the British Department of Trade and Industryspecification MPT 1375. The May 1989 version of specification MPT 1375is incorporated herein by reference.

BACKGROUND

In a cordless telephone system, it is necessary to provide a way ofcarrying the signal for the contents of communication, normally speech,in both directions between parts of the system which are not connectedby a cord or wire. Additionally, it will normally be necessary to passother signals between the parts, which are used to control the operationof the parts or carry other control messages separate from the contentof the communication. In some known radio telephone systems, therequirement for two way communication is achieved by providing two radiochannels between the parts, each channel being used for communication inone respective direction. In an embodiment of the present invention,multiplexed signal structures are provided enabling a plurality oflogical channels to be carried with communication in both directions,over a single signal communications channel. In the embodiment, the useof different multiplex structures at different times permits differencesin the logical channel structure of the cordless communication atdifferent stages of the creation and use of the cordless communicationslink.

In a conventional radio telephone system, an arrangement must beprovided enabling a link to be set up so that parts can communicate witheach other. When one of the parts is operating in a manner synchronisedto some routine, it may be difficult for another part to establish alink with the first part if the second part is not itself synchronisedto the same routine. In an embodiment of the present invention, anarrangement is provided allowing for asynchronous initiation of a linkbetween two parts, even when one of the parts is operating in asynchronous manner.

In a radio telecommunication system, the ability of a radio link tocarry useful signals will tend to vary in accordance with externalfactors, such as interference and transmission past obstructions.Accordingly, it is advantageous to encode transmitted signals for errordetection and correction and/or monitor the link quality to enableremedial steps such as breaking and re-establishing the link, possiblyon a different radio channel, if the link quality becomes unacceptablylow. In an embodiment of the present invention, an arrangement isprovided in which two logical channels are multiplexed together, withsignals of one logical channel being encoded to enable error detection,and detected errors in this logical channel being monitored and used asa measure of the extent to which the other channel is exposed to errors.

In a system for radio telecommunications, there will typically be alarge number of communication devices capable of communicating in thesystem, some of which may be more sophisticated and have greatercommunication abilities than others. In order for two devices tocommunicate with each other, they must communicate in a manner which iswithin the capabilities of both devices. Thus, when a relativelysophisticated device communicates with an unsophisticated device, theymust communicate in a manner within the capabilities of theunsophisticated device. However, it is inefficient to force thesophisticated device also to communicate in this particular manner whenit is communicating with another sophisticated device capable ofcommunicating in a different manner. In an embodiment of the presentinvention, devices conduct an operation (sometimes referred to as a"negotiation" operation) during the creation of a cordlesstelecommunications link, so as to adopt a way of communicating which iswithin the capabilities of both devices.

When two devices are communicating over a cordless telecommunicationslink, it may be necessary for the operations of the devices to besynchronised with each other and this may be done by one devicerecognising a particular part of a signal transmitted by the otherdevice, the signal part having a predetermined timing. In this case,incorrect synchronisation can arise if the receiving part incorrectlyidentifies a different part of the transmitted signal as the part to berecognised for synchronisation. In an embodiment of the presentinvention, a signal is transmitted having a data structure such that aportion used for synchronisation has a low correlation with otherportions of the signal not containing the synchronisation part.Additionally, in the embodiment the synchronisation part has a lowcorrelation with time-shifted versions of itself. Preferably, signalparts used for synchronisation are transmitted in both directionsbetween the devices, and a signal part used for synchronisation andtransmitted in one direction is arranged to have a low correlation witha signal part used for synchronisation and transmitted in the otherdirection.

When a plurality of devices capable of communicating over a cordlesscommunications link are present in the same area, and several arescanning communication channels to detect another device seeking to setup a communications link, there is a possibility that two devices maydetect the same request for a communications link on a channel and bothrespond to the request simultaneously. The resulting interference on thechannel may result in neither device establishing the communicationslink. If the subsequent behaviour of the two devices in scanning thechannels for requests for a communications link is identical, such asimultaneous response and interference is likely to occur with everysubsequent detection of a communication request. In an embodiment of thepresent invention, some devices are arranged so as to have a behaviourfollowing such a simultaneous response and interference which isdifferent from each other, so as to reduce the likelihood of subsequentrepetitions of the simultaneous response and interference.

Devices communicating with each other over a cordless telecommunicationslink may exchange "handshake" signals to confirm that communicationbetween them over the link is still taking place successfully. If one ofthe devices fails to receive a handshake signal within a certain period,it may conclude that the link has been broken. However, the device whichhas ceased to receive handshake signals will typically still betransmitting them until the end of the period at which it concludes thatthe link has been broken. If these handshake signals are successfullyreceived by the other device, then the other device will not becomeaware of the failure of the link until a further period after the firstdevice ceases to transmit handshake signals. Thus, when a transmissionlink fails in one direction only, the reaction of the devices may bedelayed and will typically not be synchronised with each other. In anembodiment of the present invention, if a device fails to receive ahandshake signal within a first period of its most recent receipt of ahandshake signal, it concludes that the link has been lost. In themeantime, it continues to transmit handshake signals, but if it has notreceived a handshake signal within a second, shorter, period since themost recent handshake signal, it transmits a signal indicating that itis failing to receive handshake signals. Thus, if a link breaks down inone direction only, the device which is continuing to receivetransmissions is rapidly notified that the other device has ceased toreceive transmissions, and the link re-establishment actions of thedevices can be better co-ordinated.

Where two devices communicate with each other in a synchronised manner,it is possible for the transmission of information to become corruptedby a loss of synchronisation, even though the transmission quality ofthe communication link may be unimpaired. In an embodiment of thepresent invention, some of the transmitted information is coded toenable error detection, and the detection of errors in this data can beused as an indication that synchronisation between the devices has beenlost.

When two devices are communicating with each other over a communicationslink, in a synchronised manner, one of them may be designated asynchronisation master, and the other a synchronisation slave, such thatthe slave is required to synchronise itself to the operations of themaster. If the link fails, or for any other reason the devices arerequired to break and re-establish the link, re-establishment may bedifficult if the slave device ceases to be synchronised to the masterand therefore fails to detect link re-establishment signals from themaster. In an embodiment of the present invention, when linkre-establishment is required the initial signalling to carry out there-establishment is always transmitted by the slave device.

If one of the devices in a link is portable or mobile, the link may bebroken by the movement of that device. It may then be impossible tore-establish the link between the same two devices. If one of thedevices is also an endpoint of a communications path, e.g. a handset,and the other is only a relay station, e.g. a base station linked to acommunications network, it is preferable to re-establish the link usingthe same endpoint device, but possibly a different relay device, for theconvenience of the user. However, it may be difficult for thecommunications network to monitor which relay device an endpoint deviceis near at any given time. In an embodiment of the present invention theendpoint device transmits the initial signals in link re-establishment,and the link can be re-established using any relay device which receivesthe transmissions. The endpoint device is typically the mobile device,e.g. a portable telephone handset.

When devices are communicating over a communications link using analternating transmission burst arrangement, there may be a failure ofcommunication if the timings of the transmissions of the two devices arenot properly co-ordinated and their transmissions partially overlapinstead of alternating correctly. In an embodiment of the presentinvention one device derives the timing for the transmission of a burstfrom the time at which it receives a burst from the other device.

When devices communicate over a telecommunications link, it may benecessary to transmit signals belonging to a logical channel for thepurposes of link maintenance, even though there is no information to betransmitted in that channel at that time. If a random signal is sent inthat logical channel under the circumstances, it may by chance resemblesome meaningful signal transmitted over the communications link,resulting in incorrect operation of the device receiving the signal. Inan embodiment of the present invention, a specified signal structure isprovided for a logical channel which structure conveys no usefulinformation in that channel, but which is chosen not to resemble asignal the reception of which could cause incorrect operation of thereceiving device.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention there is provideda telecommunication system in which first and second devices are capableof synchronous time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

first and second formats of digital data in the said bursts are used insaid time-division two-way communication, both of which formats includeinformation for a first logical communication channel which carriessignalling data such as device identification codes and instructionsfrom one device to the other;

the first said format also includes information for a second logicalcommunication channel which carries data, such as digitally encodedspeech, which is to be communicated between the devices; and

the second said format also includes a synchronisation pattern notincluded in the first format, the synchronisation pattern enabling asaid device to determine the timing of the bursts received by it fromthe other said device.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform synchronous time-division two-way communication with each otherover a radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

first and second formats of digital data in the said bursts are used insaid time-division two-way communication, both of which formats includeinformation for a first logical communication channel which carriessignalling data such as device identification codes and instructionsfrom one device to the other;

the first said format also includes information for a second logicalcommunication channel which carries data, such as digitally encodedspeech, which is to be communicated between the devices; and

the second said format also includes a synchronisation pattern notincluded in the first format, the synchronisation pattern enabling asaid device to determine the timing of the bursts received by it fromthe other said device.

Preferably, in the said first format, information for the first logicalcommunication channel is transmitted both before and after informationfor the second logical communication channel.

Preferably, in the said first format, the same number of bits ofinformation for the first channel are transmitted before the informationfor the second channel as are transmitted after the information for thesecond channel.

Preferably, in the said second format, information for the first logicalcommunication channel is transmitted both before and after the saidsynchronisation pattern.

Preferably, in the said second format, the same number of bits ofinformation for the first channel are transmitted before the saidsynchronisation pattern as are transmitted after the saidsynchronisation pattern.

Preferably, more bits of information for the first channel aretransmitted in a burst of the second format than in a burst of the firstformat.

Preferably, more bits of information for the second channel aretransmitted in a burst of the first format than the number of bits ofinformation for the first channel in a burst of either the first or thesecond format.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of synchronous time-division two-way communication with eachother over a radio channel by exchanging radio signals in alternatingbursts carrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

the said second device is also capable of transmitting asynchronously aburst comprising one or more portions of digital data for a logicalcommunication channel, which carries signalling data such as deviceidentification codes, followed by one or more further portions ofdigital data, after which the second device ceases transmission for aperiod to enable it to receive a reply from the first device, eachrespective said portion or further portion of digital data comprising aplurality of occurrences of a respective digital data sequence, and thedigital data sequence for each said further portion of digital dataproviding a synchronisation pattern to enable the said first device todetermine the timing of the burst when it receives it.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at a time when the said first and second devices are not performing thetime division two-way communication, the second device transmitsasynchronously a burst comprising one or more portions of digital datafor a logical communication channel, which carries signalling data suchas device identification codes, followed by one or more further portionsof digital data, after which the second device ceases transmission for aperiod to enable it to receive a reply from the first device, eachrespective said portion or further portion of digital data comprising aplurality of occurrences of a respective digital data sequence, and thedigital data sequence for each said further portion of digital dataproviding a synchronisation pattern to enable the said first device todetermine the timing of the burst when it receives it.

Preferably, the said bursts exchanged in the said time division two-waycommunication are exchanged in a succession of burst periods all of thesame length, a burst being transmitted from the first device to thesecond device and a burst being transmitted from the second device tothe first device at different times in a burst period, and each saidportion or further portion of digital data in a said asynchronouslytransmitted burst lasting for at least the length of a said burstperiod, and each said digital data sequence lasting for no more thanhalf the length of a part of a said burst period in which part the firstdevice does not transmit a burst.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

in bursts of a first type information is transmitted for both a firstlogical communication channel, which carries signalling data such asdevice identification codes and instructions from one device to theother, and a second logical communication channel, which carries data tobe communicated between the devices such as digitally encoded speech,

and the first and second devices conduct an operation to select, inaccordance with their capabilities, a format for bursts of the firsttype from a predefined set of formats comprising first and secondformats which differ in the amount of information for the first logicalcommunication channel carried in each burst.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

in bursts of a first type information is transmitted for both a firstlogical communication channel, which carries signalling data such asdevice identification codes and instructions from one device to theother, and a second logical communication channel, which carries data tobe communicated between the devices such as digitally encoded speech,

and the first and second devices conduct an operation to select, inaccordance with their capabilities, a format for bursts of the firsttype from a predefined set of formats comprising first and secondformats which differ in the amount of information for the first logicalcommunication channel carried in each burst.

Preferably, the said alternating bursts are transmitted in burstperiods, one burst being transmitted in each direction at differenttimes in a burst period, and the said first and second formats requiredifferent lengths of time for transmission of a burst but the samelength of time for a burst period.

Preferably, the said first and second formats carry the same amount ofthe second logical communication channel per burst as each other.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least some bursts comprise information for a first logical channeland information for second logical channel at different times in theburst, the data of the first logical channel being structured so as toenable the detection of transmission errors, and each said device usingdetected errors in the first logical channel in bursts received by it asan indication of the quality of transmission of the second logicalchannel.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least some bursts comprise information for a first logical channeland information for second logical channel at different times in theburst, the data of the first logical channel being structured so as toenable the detection of transmission errors, and each said device usingdetected errors in the first logical channel in bursts received by it asan indication of the quality of transmission of the second logicalchannel.

Preferably, information for the first logical channel is provided in asaid burst both before and after information for the second logicalchannel.

Preferably, if the quality of transmission of the second logicalchannel, as determined by a said device using detected errors in thefirst logical channel, fails to meet a predetermined criterion, thedevice enters a mode to re-establish the said time-division two-waycommunication.

Preferably, the device which determines the failure of the quality oftransmission of the second logical channel to meet the predeterminedcriterion sends a message to the other device before it enters the saidmode, to inform the other device that it is about to enter the saidmode.

Preferably, the said devices do not structure the data of the secondlogical channel to enable the detection of transmission errors.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

each of the first and second devices sends repeatedly one of a pre-setgroup of signal codes while they are exchanging the said radio signals,at a rate such that the period between successive transmissions by thesame device of one of the pre-set group of codes does not exceed a firstpredetermined length of time, a first code of said group of codesnormally being sent if the sending device has received any of said groupof codes within the first predetermined length of time before the timeit sends the code, and a second code of said group of codes normallybeing sent otherwise, and each of the first and second devices enteringa mode for re-establishing the time-division two-way communication if ithas not received the said first code for a second predetermined lengthof time greater than the first.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

each of the first and second devices sends repeatedly one of a pre-setgroup of signal codes while they are exchanging the said radio signals,at a rate such that the period between successive transmissions by thesame device of one of the pre-set group of codes does not exceed a firstpredetermined length of time, a first code of said group of codesnormally being sent if the sending device has received any of said groupof codes within the first predetermined length of time before the timeit sends the code, and a second code of said group of codes normallybeing sent otherwise, and each of the first and second devices enteringa mode for re-establishing the time-division two-way communication if ithas not received the said first code for a second predetermined lengthof time greater than the first.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which a remote unit is capable oftime-division two-way communication with a base station over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the remote unit and the basestation is completed before transmission of the next burst by the otherof the remote unit and the base station is begun, to enable the remoteunit to communicate via the base station with a further device,

characterised in that:

if either the remote unit or the base station with which it is in saidtime-division two-way communication decides that re-establishment of thetime-division two-way communication is required, it takes steps to causethe remote unit to transmit radio signals to initiate the saidre-establishment.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which a remote unit and a basestation perform time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the remote unitand the base station is completed before transmission of the next burstby the other of the remote unit and the base station is begun, to enablethe remote unit to communicate via the base station with a furtherdevice,

characterised in that:

if either the remote unit or the base station with which it is in saidtime-division two-way communication decides that re-establishment of thetime-division two-way communication is required, it takes steps to causethe remote unit to transmit radio signals to initiate the saidre-establishment.

Preferably, the handset is capable of the said time-division two-waycommunication with a plurality of base stations, so that the saidre-establishment takes place between the said remote unit and a saidbase station which is not necessarily the same base station as theremote unit was previously in said communication with.

Preferably, the said radio signals transmitted by the remote unit toinitiate the said re-establishment comprise the radio signalstransmitted by the remote unit to initiate the establishment of saidcommunication when the remote unit has not been in said communicationimmediately beforehand, modified so as to convey an identification ofthe time-division two-way communication to be re-established.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least some of the information communicated between the devices over afirst logical channel is structured in words which include an errordetection code, and a predetermined word is defined for communicationbetween the devices which includes a said error detection code butcarries substantially no messages from the transmitting device to thereceiving device.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least some of the information communicated between the devices over afirst logical channel is structured in words which include an errordetection code, and a predetermined word is defined for communicationbetween the devices which includes a said error detection code butcarries substantially no messages from the transmitting device to thereceiving device.

Preferably, the transmission of a first type of said words is precededby the transmission over the first logical channel of a set patternindicating the timing of the following word, and no part of thepredetermined word includes data in the same sequence as the said setpattern.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least some of the information communicated between the devices over afirst logical channel is structured in words, and transmission of afirst type of said words is preceded by transmission over the firstlogical channel of a set pattern indicating the timing of the followingword,

and in between repetitions of words of the first type carrying the samemessages or parts of messages and having the same data sequence there istransmitted over the first logical channel either a different word ofthe first type or a predefined sequence which does not include the saidset pattern, whereby if the data sequence of a repeated said word of thefirst type includes the said set pattern it is repeated sufficientlyinfrequently to allow the receiving device to identify the said setpattern correctly in the transmission of the set pattern before the nexttransmission of the repeated word even if the receiving deviceincorrectly identifies the set pattern in the data sequence of theprevious transmission of the repeated word as an occurrence of the setpattern preceding a word of the first type.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least some of the information communicated between the devices over afirst logical channel is structured in words, and transmission of afirst type of said words is preceded by transmission over the firstlogical channel of a set pattern indicating the timing of the followingword,

and in between repetitions of words of the first type carrying the samemessages or parts of messages and having the same data sequence there istransmitted over the first logical channel either a different word ofthe first type or a predefined sequence which does not include the saidset pattern, whereby if the data sequence of a repeated said word of thefirst type includes the said set pattern it is repeated sufficientlyinfrequently to allow the receiving device to identify the said setpattern correctly in the transmission of the set pattern before the nexttransmission of the repeated word even if the receiving deviceincorrectly identifies the set pattern in the data sequence of theprevious transmission of the repeated word as an occurrence of the setpattern preceding a word of the first type.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which a device of a first type iscapable of time-division two-way communication with any one of aplurality of devices of a second type over a radio channel by exchangingradio signals in alternating bursts carrying digital data, such thatduring the time-division two-way communication transmission of a saidburst from one of the devices is completed before transmission of thenext burst by the other of the devices is begun,

characterised in that:

at least some of the bursts contain a synchronisation pattern which maybe detected asynchronously by the receiving device to enable it todiscover the timing of the received burst, the device of the first typetransmitting a first synchronisation pattern or one of a group of firstsynchronisation patterns and the devices of the second type transmittinga second synchronisation pattern or one of a group of secondsynchronisation patterns, the first synchronisation pattern or patternsbeing different from the second synchronisation pattern or patterns andthe devices of the second type not responding to reception of the or asecond synchronisation pattern, whereby devices of the second type donot respond to reception of transmissions by other devices of the secondtype.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which a device of a first typeand any one of a plurality of devices of a second type performtime-division two-way communication with each other over a radio channelby exchanging radio signals in alternating bursts carrying digital data,such that during the time-division two-way communication transmission ofa said burst from one of the devices is completed before transmission ofthe next burst by the other of the devices is begun,

characterised in that:

at least some of the bursts contain a synchronisation pattern which maybe detected asynchronously by the receiving device to enable it todiscover the timing of the received burst, the device of the first typetransmitting a first synchronisation pattern or one of a group of firstsynchronisation patterns and the devices of the second type transmittinga second synchronisation pattern or one of a group of secondsynchronisation patterns, the first synchronisation pattern or patternsbeing different from the second synchronisation pattern or patterns andthe devices of the second type not responding to reception of the or asecond synchronisation pattern, whereby devices of the second type donot respond to reception of transmissions by other devices of the secondtype.

Preferably, the said time-division two-way communication may beperformed by any one of a plurality of devices of the first type, andthe devices of the first type do not respond to reception of the or afirst synchronisation pattern, whereby devices of the first type do notrespond to transmissions by other devices of the first type.

Preferably, each said device transmits a predetermined synchronisationpattern while attempting to initiate communication by said radio signalswith a device of the other type, and subsequently transmits a differentpredetermined synchronisation pattern after said communication has beeninitiated.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least some of the bursts transmitted by a said device contain asynchronisation pattern of bits of data, which may be detectedasynchronously by the receiving device to enable it to discover thetiming of the transmitted burst, and also contain bits of variable data,and the receiving device deems the synchronisation pattern to be presentin the received data when a comparison operation between the receiveddata and a stored copy of the synchronisation pattern results in no morethan K bits of the received data failing the comparison, where K is zeroor a positive integer, and the arrangement of bits in each said burstbeing such that in any consecutive string of L bits of data in theburst, there are less than L-K bits of variable data.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least some of the bursts transmitted by a said device contain asynchronisation pattern of bits of data, which may be detectedasynchronously by the receiving device to enable it to discover thetiming of the transmitted burst, and also contain bits of variable data,and the receiving device deems the synchronisation pattern to be presentin the received data when a comparison operation between the receiveddata and a stored copy of the synchronisation pattern results in no morethan K bits of the received data failing the comparison, where K is zeroor a positive integer, and the arrangement of bits in each said burstbeing such that in any consecutive string of L bits of data in theburst, there are less than L-K bits of variable data.

Preferably, the arrangement of bits in each said burst is such that inany consecutive string of L bits of data in the burst, there are no morethan L-K-6 bits of variable data.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises a first portionhaving a repeating pattern of fixed value and variable bits and a secondportion comprising an L-bit synchronisation pattern which may bedetected asynchronously by the receiving device to enable it to discoverthe timing of the burst, and the receiving device deems thesynchronisation pattern to be present in the received data when acomparison operation between the received data and a stored copy of thesynchronisation pattern results in no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer,

the L-bit synchronisation pattern and the repeating pattern of fixedvalue and variable bits being selected such that a string of Lsuccessive bits of the repeating pattern, starting at any position in arepeat of the pattern, matches less than L-K bits of the synchronisationpattern even if it is assumed that every variable bit in the stringprovides a match.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises a first portionhaving a repeating pattern of fixed value and variable bits and a secondportion comprising an L-bit synchronisation pattern which may bedetected asynchronously by the receiving device to enable it to discoverthe timing of the burst, and the receiving device deems thesynchronisation pattern to be present in the received data when acomparison operation between the received data and a stored copy of thesynchronisation pattern results in no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer,

the L-bit synchronisation pattern and the repeating pattern of fixedvalue and variable bits being selected such that a string of Lsuccessive bits of the repeating pattern, starting at any position in arepeat of the pattern, matches less than L-K bits of the synchronisationpattern even if it is assumed that every variable bit in the stringprovides a match.

Preferably, any said string of L successive bits of the said repeatingpattern matches no more than L-K-2 bits of the synchronisation patterneven if it is assumed that every variable bit of the string provides amatch.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises an L-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, andthe receiving device deems the synchronisation pattern to be present inthe received data when a comparison operation between the received dataand a stored copy of the synchronisation pattern results in no more thanK bits of the received data failing the comparison, where K is zero or apositive integer,

the synchronisation pattern being adjacent a portion of the burst madeup of fixed value bits, and the number of matches between thesynchronisation pattern and any string of L successive bits of the burstcomposed only of least a part of the said portion of fixed value bitsand an adjacent part of the synchronisation pattern being less than L-K.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises an L-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, andthe receiving device deems the synchronisation pattern to be present inthe received data when a comparison operation between the received dataand a stored copy of the synchronisation pattern results in no more thanK bits of the received data failing the comparison, where K is zero or apositive integer,

the synchronisation pattern being adjacent a portion of the burst madeup of fixed value bits, and the number of matches between thesynchronisation pattern and any string of L successive bits of the burstcomposed only of least a part of the said portion of fixed value bitsand an adjacent part of the synchronisation pattern being less than L-K.

Preferably, different said L-bit synchronisation patterns are used underdifferent circumstances, and the number of matches between any saidsynchronisation pattern and any string of L successive bits of the burstcomposed only of any other said synchronisation pattern or at least apart of the said portion of fixed value bits and an adjacent part of anyother said synchronisation pattern is less than L-K.

Preferably, for at least some of the said synchronisation patterns, thesaid number of matches does not exceed L-K-8.

Preferably, for all of the said synchronisation patterns, the saidnumber of matches does not exceed L-K-7.

Preferably, K is not zero and more preferably, K is two.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises an L-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, thesynchronisation pattern having a peak self-correlation side lobe valueof not more than +2, for any amount of offset, where theself-correlation side lobe value at an amount of offset is defined asthe number of matches between bits of the pattern and itself offset bythe amount, minus the number of mismatches between the bits of thepattern and itself at the same amount of offset.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises an L-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, thesynchronisation pattern having a peak self-correlation side lobe valueof not more than +2, for any amount of offset, where theself-correlation side lobe value at an amount of offset is defined asthe number of matches between bits of the pattern and itself offset bythe amount, minus the number of mismatches between the bits of thepattern and itself at the same amount of offset.

In accordance with another aspect of the present invention there isprovided a telecommunication system in which first and second devicesare capable of time-division two-way communication with each other overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication transmission of a said burst from one of the first andsecond devices is completed before transmission of the next burst by theother of the first and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises a 24-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, thesynchronisation pattern, when given in hexadecimal format, being one of:BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728.

In accordance with another aspect of the present invention there isprovided a method of telecommunication in which first and second devicesperform time-division two-way communication with each other over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communicationtransmission of a said burst from one of the first and second devices iscompleted before transmission of the next burst by the other of thefirst and second devices is begun,

characterised in that:

at least one format of digital data in a burst comprises a 24-bitsynchronisation pattern which may be detected asynchronously by areceiving device to enable it to discover the timing of the burst, thesynchronisation pattern, when given in hexadecimal format, being one of:BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728.

The present invention also includes a communication device usable in anyof the above systems, and in particular a communication device usable asthe first device and a communication device usable as the second device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention, given by way of example, will nowbe described with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of a telecommunications system connected to abase station of a system embodying the present invention;

FIG. 2 is a schematic illustration of the pattern of burst transmissionsin an embodiment of the present invention;

FIG. 3 is a schematic representation of the frequency and amplitudevariation of the radio frequency signal in a burst in an embodiment ofthe present invention;

FIGS. 4a and 4b illustrate schematically first and second versions of afirst type of data structure in a signal burst used in an embodiment ofthe present invention;

FIG. 5 illustrates schematically a second type of data structure in asignal burst used in an embodiment of the present invention;

FIG. 6 illustrates schematically the relative timing of a third type ofdata structure, transmitted by the handset in an embodiment of thepresent invention, relative to the transmission cycle of a base stationin the embodiment of the present invention;

FIG. 7 illustrates in more detail a portion of FIG. 6;

FIG. 8 illustrates schematically the arrangement of data in the datastructure of FIG. 6;

FIG. 9 illustrates in more detail part of FIG. 8;

FIG. 10 illustrates a first version of a handset for use in anembodiment of the present invention;

FIG. 11 illustrates a second version of a handset used in an embodimentof the present invention;

FIG. 12 illustrates schematically the parts of a handset in anembodiment of the present invention;

FIG. 13 illustrates schematically a version of a base station for use inan embodiment of the present invention;

FIG. 14 illustrates schematically the parts of a base station for use inan embodiment of the present invention;

FIG. 15 is a schematic block diagram of the control circuit of ahandset;

FIG. 16 is a schematic block diagram of the control circuit of the basestation;

FIG. 17 is a schematic block diagram of the programmable multiplexer ofFIGS. 15 and 16;

FIG. 18 is a schematic block diagram of the programmable demultiplexerof FIGS. 15 and 16;

FIG. 19 is a schematic block diagram of the system controller of FIGS.15 and 16;

FIG. 20 is a schematic block diagram of the S channel controller ofFIGS. 15 and 16;

FIG. 21 is a flow diagram for the setting up of a link from a basestation to a handset;

FIG. 22 is a schematic representation of the sequence of signalstransmitted when a link is set up from a base station to a handset;

FIG. 23 is a flow diagram for the setting up of a link from a handset toa base station;

FIG. 24 is a schematic representation of the sequence of signalstransmitted when a link is set up from a handset to a base station;

FIG. 25 is an overall view of the structure of data in the D channel;

FIG. 26 shows schematically how D channel code words may be assembledinto packets and packets into messages;

FIG. 27 illustrates schematically the general format of a D channel codeword;

FIG. 28 illustrates schematically the format of a fixed format typeaddress code word of the D channel;

FIG. 29 illustrates schematically the format of a variable format typeaddress code word of the D channel;

FIG. 30 illustrates schematically the format of a data code word of theD channel;

FIG. 31 illustrates schematically the structure of a fixed lengthmessage in the D channel;

FIG. 32 illustrates schematically the structure of a variable lengthmessage in the D channel;

FIG. 33 illustrates schematically the sequence of handshake signalsleading to link re-establishment when handshake is lost only from ahandset to a base station;

FIG. 34 illustrates schematically the sequence of handshake signalsleading to link re-establishment when handshake is lost only from a basestation to a handset;

FIG. 35 is a flow diagram of the link quality monitoring process carriedout in a handset or a base station;

FIG. 36 illustrates schematically the structure of a FILL-IN word in theD channel;

FIG. 37 is a view similar to FIG. 1, illustrating alternative types ofbase station for use in embodiments of the present invention;

FIG. 38 illustrates in more detail a first alternative structure for abase station embodying the present invention;

FIG. 39 illustrates in more detail a second alternative structure for abase station embodying the present invention;

FIG. 40 illustrates a third alternative structure for a base stationembodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this specification, "kbit" and "kword" will be used as abbreviationsfor "kilobits" and "kilowords", meaning "thousand bits" and "thousandwords" respectively, in accordance with normal practice in the digitalsignalling art.

Overview

FIG. 1 shows schematically a number of telecommunications devicesconnected to a telecommunications network 1. The telecommunicationsnetwork 1 is typically a PSTN (Public Switched Telephone Network),although in the future an ISDN (Integrated Services Digital Network) maybe more common. The telecommunications network 1 is connected to avariety of devices, and as examples of these FIG. 1 shows base stations3 of a cordless telephone apparatus embodying the present invention, atelephone and other telecommunications apparatus 7 such as a facsimilemachine or a modem for a computer. Each of these communications devicesis connected to the telecommunications network 1 by a network link 9,which will typically include some or all of a wired link, an opticalfibre link and a long distance radio link.

As shown in FIG. 1, the cordless telephone apparatus embodying thepresent invention comprises a base station 3 and a remote unit 11. Theremote unit will typically be a telephone handset for speechcommunication, and will be referred to hereinafter as a handset. Thehandset 11 communicates with the base station 3 over a radio link,diagrammatically represented at 13 in FIG. 1, and this provides accessfor the user of the handset 11 to the telecommunications network 1.

Although FIG. 1 shows only one handset 11 per base station 3,alternative embodiments will be referred to with respect to otherfigures, in which a plurality of handsets 11 are associated with onebase station 3, either so that the base station 3 can communicate withany chosen one of the handsets 11, or in some cases so that the basestation 3 can communicate simultaneously with different handsets 11 ondifferent radio channels. The typical uses of such cordless telephonesystems include:

the provision of public telepoint services, in which carriers ofhandsets 11 can go to the vicinity of a publicly available base station3, set up a radio link with the base station 3, and thereby access thetelecommunications network 1;

the provision of an office private branch exchange and/or intercomsystem, having separately numbered extensions; and

the provision of a domestic or office extension and/or intercom systemin which extensions are not individually numbered.

For purely intercom use, there would, of course, be no need for the basestation 3 to be connected to a telecommunications network 1.

The construction and operation of the cordless telephone systemcomprising the base station 3 and the handset 11 will now be described.The system is in accordance with the British Department of Trade andIndustry specification MPT 1375, incorporated herein by reference, towhich reference may be had for further details and regulatoryrestrictions. In MPT 1375, the base station 3 is normally referred to as"the cordless fixed part" or "the CFP", and the handset 11 is normallyreferred to as "the cordless portable part" or "the CPP", and whereappropriate similar terminology will be used herein. The terms "fixed"and "portable" refer to the nature of the typical connections madebetween the parts and the telecommunications network 1, and do notnecessarily mean that the base station 3 cannot move from place toplace.

Burst Mode Transmission and Timing

The base station 3 and the handset 11 communicate with each other over asingle radio channel, using time division duplex burst modetransmission. The pattern of transmissions in an established radio linkis illustrated in FIG. 2. The top line in FIG. 2 representstransmissions by the base station 3, and the lower line representstransmissions by the handset 11.

A complete period in the burst mode transmission system lasts for twomilliseconds. Two bursts of data, one in each direction, are transmittedin each burst period. Each two millisecond burst period begins at thetime t1 in FIG. 2, when the base station 3 begins to transmit a burst.The burst transmission from the base station 3 ends at time t2, which isslightly less than 1 millisecond after time t1. After a slight gap, thehandset 11 begins a transmission burst at time t3, and this transmissionburst also lasts for slightly less than 1 millisecond and ends at timet4. After another slight gap, the next 2 millisecond transmission periodbegins.

The length of the transmission periods is controlled by the base station3, which ensures that successive times t1, at which it beginstransmitting a burst, are 2 milliseconds apart. The base station 3 andthe handset 11 both transmit digital data at 72 kilobits per second.Therefore each 2 ms burst period is equivalent to 144 bit periods.Depending on the burst structure being used, as will be described later,each burst comprises either 68 bits or 66 bits. Thus, the period from t1to t2 (and also the period from t3 to t4) is either about 0.9167 ms orabout 0.9444 ms.

The gap between time t2 and time t3, and the gap between time t4 and thefollowing time t1, are present to give the base station 3 and thehandset 11 time to switch between transmitting and receiving modes, andalso to allow for RF propagation delay of the signals. The size of thegap between time t2 and time t3 is determined by the handset 11. If thetwo bursts in the burst period are each 66 bits long, the dset 11 willbegin transmitting the first bit of its burst 5.5 bit periods after theend of the last bit of the received burst from the base station 3. Ifthe two bursts are each 68 bits, the handset waits for only 3.5 bitperiods. Assuming that there is no RF propagation delay of the signals,the gap between time t4 and the following time t1 will be 1 bit periodgreater than the gap between time t2 and time t3. If there is an RFpropagation delay in the transmission of the signals, the handset 11will receive the transmission from the base 3 slightly later, andaccordingly the transmission from the handset 11 will begin slightlylater, and the gap between time t4 and the following time t1 isaccordingly reduced. The system can cope with a cumulative propagationdelay of 2 bit periods in each burst period (typically up to 1 bitperiod in each direction), while still allowing the base station 3 atleast 2.5 bit periods to switch from receiving to transmitting, evenwhen 68-bit bursts are being used. The base station 3 is assumed to beable to switch faster than the handset 11 (which is allowed at least 3.5bit periods), to allow simpler and lower cost circuitry to be used inthe handset 11.

As will be explained later, communication between the base station 3 andthe handset 11 may use more than one data structure for the bursts.68-bit bursts can only occur in one data structure, known as multiplex1, whereas a second data structure known as multiplex 2 always uses66-bit bursts. During a change between communication using multiplex 1and communication using multiplex 2, it is possible for there to be abrief period during which one part is transmitting 68-bit multiplex 1bursts while the other is transmitting 66-bit multiplex 2 bursts. Inthis case, the gaps between times t2 and t3 and between times t4 and thenext t1 will alter correspondingly.

In addition to radio signal (RF) propagation delays, there may be signaldelays through the circuitry of the base station 3 and the handset 11.As these are internal to the devices, they can be compensated for in thedesign of the devices. These delays are not included in FIG. 2 which isconcerned only with the timing of signals at the aerials of the devices.

The digital bits are transmitted in each data burst by frequencymodulation, known as frequency shift keying (FSK), of the radio carrierfrequency.

The overall structure of a data burst is illustrated in FIG. 3. In thegap between the time when one of the base station 3 and the handset 11stops transmitting and the time when the other starts transmitting, theradio frequency will have zero amplitude. In order to transmit thedigital data of a burst, the radio frequency signal must be transmittedat a suitable amplitude. In order to avoid an amplitude modulationsplash of interference on other channels, the part which is about totransmit a burst begins to transmit the radio frequency signal beforethe beginning of the data burst, and slowly increases the amplitude ofthe signal. This is the period from a to b in FIG. 3. The RF amplitudeenvelope is shown at 15 in FIG. 3.

At instant b in FIG. 3, the first bit period of the data burst begins.One binary bit of data is transmitted in each bit period, by frequencymodulation, with the transmitted frequency being greater than thecarrier frequency for the logical "1", and being less than the carrierfrequency for a logical "0". In order to avoid a frequency modulationsplash on other channels, the transmitted frequency cannot be variedinstantaneously, and therefore it varies gradually as shown by the RFfrequency envelope 17 in FIG. 3. Instant c in FIG. 3 represents the endof the first bit period and the beginning of the second bit period.Instant d represents the end of the last bit period.

Because of the delay dispersion introduced by some types of filter usedin the base station 3 and the handset 11, it is necessary to maintainthe amplitude of the RF signal at no more than 6 dB below its amplitudeduring the data burst, for half a bit period until instant e, to ensurethat all data is correctly received and processed at the receiving end.This half bit period is known as a suffix, and is represented atreference numeral 19. Following the end of the suffix 19 at instant e,the radio frequency is reduced in amplitude gradually to avoid amplitudemodulation splash, and that ends the transmission by that part.

Data Structure of Bursts--MUX 1 and MUX 2

Communication between the base station 3 and the handset 11 takes placeover three logical channels. Since there is only one radio channelbetween the two parts of the system, the three logical channels arecombined by time division multiplexing. At any given time during anestablished link, the base station 3 and the handset 11 will becommunicating with each other using bursts as described above, having apre-selected data structure for providing time division multiplexingbetween the channels. In each of the available data structures, eachburst carries two of three logical channels, multiplexed together. Eachof these data structures is known as a multiplex, abbreviated as "MUX".

Once a link has been set up, and the bursts between the base station 3and the handset 11 are carrying the information which the user wishes toexchange with the person or device he is connected to through thetelecommunications network 1, the data structure of bursts will be aform called multiplex 1 (or MUX 1). This is shown in FIG. 4. Two formsof multiplex 1 are possible: multiplex 1.2 as shown in FIG. 4a andmultiplex 1.4 as shown in FIG. 4b.

In multiplex 1.2, each burst is 66 bits long. The first bit and the lastbit are defined as belonging to the D logical channel and the central 64bits are defined as belonging to the B logical channel. The B channelcarries the data which the user is transmitting or receiving. In thenormal case, where the handset 11 is being used to hold a telephoneconversation, the B channel will be carrying digitally encoded speechsound data.

The D channel carries signalling data. This data may represent variousthings, as will be described later, but most data carried by the Dchannel can be assigned to one of two general types. First, there isdata communicated between the base station 3 and the handset 11 purelyfor the purpose of establishing or maintaining the radio link betweenthe parts. This data includes handshake signals, identification andauthorisation codes which enable one part to recognise the other andpermit or refuse to permit a communication link between them to beestablished, etc.. The second kind of data instructs the receiving partto take some action or informs the receiving part that some action hasoccurred at the transmitting part. For instance, if the user presses akey on the handset 11, this fact will be transmitted to the base station3 by the D channel, and if the base station 3 instructs the handset 11to display a symbol or flash the display, this will be transmitted inthe D channel.

D channel data is transmitted in code words having a defined format,which will be discussed later. Because each burst in multiplex 1.2, asshown in FIG. 4a, contains only two bits of D channel data, it requiresa large number of bursts to transmit a single code word of the Dchannel. In multiplex 1.4, as illustrated in FIG. 4b, D channel data canbe transmitted at twice the rate. Each burst in this multiplex structureis 68 bits long. The first two bits and the last two bits are defined asD channel bits, and the central 64 bits are defined as B channel bits.It should be noted that in both multiplex 1.2 and multiplex 1.4, eachburst carries the same number of B channel bits and the differencebetween them is only in the number of D channel bits carried per burst.

Although multiplex 1.4 is advantageous because it permits transmissionof the D channel at twice the rate permitted by multiplex 1.2, this isachieved by making each burst in multiplex 1.4 two bits longer (i.e. 68bits instead of 66 bits) than each burst of multiplex 1.2. Withreference to FIG. 2, this means that when the two parts arecommunicating using multiplex 1.4, the gap between times t2 and t3,during which each part can switch between transmission mode andreception mode, is reduced by two bit periods from 5.5 bit periods to3.5 bit periods. The gap between time t4 and time t1 is similarlyreduced by two bit periods. Thus, communication using multiplex 1.4 canonly take place if both the base station 3 and the handset 11 arecapable of switching between transmission and reception modes in thereduced time available. A device which cannot make this switchsufficiently quickly will only be able to use multiplex 1.2.

All base stations 3 and handsets 11 in the preferred embodiment arecapable of communicating using multiplex 1.2, even if they are alsocapable of communicating using multiplex 1.4. In any particular radiolink between a base station 3 and a handset 11, both parts must use thesame version of multiplex 1. During the initial setting up of a link thetwo parts will communicate with a different data structure, known asmultiplex 2 (or MUX 2), before switching to multiplex 1, and beforeswitching to multiplex 1 the two parts will perform an operation(sometimes known as a "negotiation" operation) to determine whichversion of multiplex 1 will be used. If both parts can communicate usingmultiplex 1.4, this will be done. If either or both of the parts canonly use multiplex 1.2, then this version of multiplex 1 must be adoptedfor this radio link.

The structure of multiplex 2 is shown in FIG. 5. This data structure isused in setting up a link before communication using the B channelbegins. In the multiplex 2 data structure, no B channel is transmitted.Each multiplex 2 data burst is 66 bits long. The first 16 bits and thelast 16 bits are defined as belonging to the D channel, and the central34 bits are defined as belonging to the third logical channel called theS channel. The first 10 bits of the S channel are P bits, which form apreamble, and simply comprise an alternation between "1" and "0". Theremaining 24 bits of the S channel are W bits, and define an S channelsynchronisation word.

The S channel is used to synchronise the two parts when a radio linkbetween them is being established. Two levels of synchronisation arerequired. First, the parts must enter bit synchronisation, so that thereceiving part when decoding a received signal divides the receivedsignal into bits with the correct timing. Second, the two parts mustenter burst synchronisation, so that one part transmits while the otheris in reception mode, and then the other transmits while the first is inreception mode, to provide the alternating burst transmission structureillustrated in FIG. 2.

When a link is being established in either direction the first radiosignals which a handset 11 receives from a base station 3 will be in themultiplex 2 format, and if the base station 3 is initiating the creationof a link with a handset 11, the first signals which the base station 3receives from the handset 11 will also be in the multiplex 2 format.(When the handset 11 is initiating the link, it first transmits in afurther data structure, called multiplex 3 (or MUX 3), which will bedescribed later.) Therefore, multiplex 2 has been designed to permitrapid synchronisation between the two parts.

Most base stations 3 and handsets 11 will contain automatic frequencycontrol or automatic gain control circuits, or both. These circuitsrequire an initial period, during which the radio signal is received, toperform their control operations and establish satisfactory radioreception by the part concerned. The first 16 bits of D channel in themultiplex 2 structure provide such a period of radio transmission,permitting the automatic gain control and automatic frequency controlcircuits to settle before the 34 bits of S channel are received.

The successive bit value reversals in the "1010 . . . " preamble patternof the P bits of the S channel provide a clear definition of the timingof the bit periods, enabling the receiving part to enter bitsynchronisation with the transmitting part before the 24 bitsynchronisation word of the S channel is received. The synchronisationword has a predetermined pattern, which the receiving part is searchingfor in the received data burst. When this bit pattern is recognised, thereceiving part knows that the 24 bits under consideration form thesynchronisation word of the S channel. Since the position of this wordin the multiplex 2 structure is defined, the receiving part can thendetermine the burst timing of the received multiplex 2 data burst, andtherefore burst synchronisation can be attained.

D channel data can vary, and it is possible that by chance 24 successiveD channel bits could have the same pattern as an S channelsynchronisation word. If this was mis-identified as the S channelsynchronisation word, the receiving part would select an incorrect bursttiming. To prevent this, the D channel is split in multiplex 2 into two16 bit portions, so that the burst does not contain 24 D channel bits asan unbroken string.

As described above with reference to FIG. 2, the base station 3determines the 2 millisecond burst period by beginning the transmissionof successive data bursts from it at 2 millisecond intervals. Thehandset 11 has to adapt its reception and transmission timing to matchthe timing of the base station 3, and thus create the time divisionduplex structure of FIG. 2. Thus, in determining burst synchronisationtiming the base station 3 acts as a master and handset 11 acts as aslave. Since the base station 3 will typically contain more accurateclocks than handset 11, this is convenient for maximising the accuracyof the burst mode timing. Additionally, if a base station 3 is capableof communicating simultaneously with different handsets 11 usingdifferent radio channels, it will normally be impractical or evenimpossible for the base station 3 to maintain two such links unless thetransmission and reception timing for the two links is synchronised.Therefore, the timing of the handset 11 must be slaved to the timing ofthe base station 3.

Link Initiation and MUX 3

If the base station 3 wishes to initiate a link, it will begintransmitting in multiplex 2. If a handset 11 is switched on but is notcommunicating in a link, it will be scanning radio channels looking fora channel on which transmission is taking place. If the handset 11detects radio transmission, it will attempt to decode it on theassumption that it is multiplex 2. The handset 11 performs theseoperations asynchronously, and if the received signal is in themultiplex 2 format the preamble portion and the synchronisation word ofthe S channel will enable the handset 11 to obtain bit and burstsynchronisation with the transmitted signals. Once this synchronisationhas been obtained, the contents of the D channel can be decoded and theprocess of link initiation can begin.

A problem may arise if a handset 11 wishes to initiate a communicationslink with a base station 3. The operation of an idle base station 3,i.e. one which is waiting for a call from a handset, may be synchronisedto the operation of an active base station, i.e. one which is already incommunication with a handset. Thus the idle base station will only havea listening window during which it can receive transmissions from ahandset 11 during the times in which the active base station is in thereception mode. This restriction on the ability of an idle base stationto listen will almost inevitably be the case if the two base stationsshare a common aerial, because RF power leaking from the transmitter ofthe base station already in communication will probably swamp thereceived power from a calling handset, so that the idle base stationwould be unable to detect any handset during these periods, even if itwas in reception mode. Therefore, it must be assumed by the handset 11that a base station has a listening window of only about 1 millisecond(72 bits) in every 2 millisecond (144 bit) burst period.

A handset 11 which is not already in communication with a base station 3will not be synchronised with the base station 3, and therefore itcannot send out a link request signal having a timing synchronised tothe listening periods of the base station 3 it is trying to reach.Therefore, the handset 11 makes a link request by transmittingasynchronously in a further data structure called multiplex 3 (or MUX3). The asynchronous nature of multiplex 3 is shown in FIG. 6.

The lower line in FIG. 6 represents the timing of the operations of thebase station 3. The base station 3 will divide time into a plurality oftransmit periods 21, and will only be able to receive signals from thehandset 11 in between these transmit periods 21. The top line in FIG. 6shows the activity of a handset 11 transmitting in multiplex 3. Eachmultiplex transmission, indicated at 23 in FIG. 6, is 720 bits long, andlasts for a period of 10 milliseconds. Successive multiplex 3transmissions 23 are separated by 288 bit periods, lasting 4milliseconds, during which the handset switches from transmission modeto reception mode and listens for a reply in multiplex 2 from the basestation 3. Thus, the total burst period of multiplex 3 transmissions is1008 bit periods or 14 milliseconds. The multiplex 3 burst period isseven times as long as the burst period for multiplex 1 and multiplex 2,and the multiplex 3 transmission lasts for five multiplex 1 or multiplex2 burst periods.

As shown in FIG. 6, each multiplex 3 transmission 23 is divided intofive equal sub-multiplex sections. Each is 144 bits long, and lasts for2 milliseconds.

Thus, each sub-multiplex lasts for the same length of time as a periodof operation of the base station 3 including a transmission period and areception period.

FIG. 7 shows a portion of FIG. 6 on an enlarged scale. Eachsub-multiplex of multiplex 3 is in turn divided into four equalrepetition periods. The data transmitted in one sub-multiplex isrepeated in each repetition period. Each repetition period contains 36bits and lasts for 0.5 milliseconds. Because the hand set 11 is notsynchronised with the base station 3, the timing of the signal receptionwindows in the operation of the base station 3 with respect to themultiplex 3 structure cannot be predicted. However, each 36 bitrepetition period of a sub-multiplex is sufficiently short that the basestation 3 must receive at least one complete repetition period of asub-multiplex in the period between successive base station transmissionperiods 21. In FIG. 7, the relative timings of the operation of the basestation 3 and the handset 11 are such that the third repetition periodof each sub-multiplex falls entirely within a base station receptionperiod, and will therefore be received by the base station 3.

Because each repetition period contains a complete copy of the databeing transmitted in one sub-multiplex period of a multiplex 3 burst, itis only necessary for the base station 3 to receive one repetitionperiod of each sub-multiplex to receive the data transmitted by thehandset 11. As shown in FIG. 6, the first four sub-multiplex periods ofmultiplex 3 are defined as carrying the D channel, while the finalsub-multiplex period of multiplex 3 is defined as carrying the Schannel. This maximises the likelihood that the base station 3 willreceive and decode a repetition period of the S channel.

Each repetition period of the S channel sub-multiplex of multiplex 3contains an S channel synchronisation word. When the base station 3detects this, it knows that there should be no multiplex 3 transmissionsduring the next two reception periods, and then in successive receptionperiods it should receive four portions of D channel and then an Schannel portion including the synchronisation word again. In this way,the base station 3 can be temporarily synchronised with the multiplex 3timing from the hand set 11, without altering its own burstsynchronisation for multiplex 1 and multiplex 2 signals.

Following decoding of the multiplex 3 signals received from the handset11, the base station can reply in multiplex 2 during the 288 bit spacebetween successive multiplex 3 transmissions. This space is sufficientlylong that it can be guaranteed to include at least one complete basestation transmit period 21, as can be seen in FIG. 6. Once the handset11 receives a reply in multiplex 2 from the base station 3, it ceases totransmit in multiplex 3 and transmits instead in multiplex 2,synchronised to the timing of the base station 3.

The arrangement of data in the multiplex 3 structure is shown in moredetail in FIGS. 8 and 9. In FIG. 8 each line represents onesub-multiplex period. The first four lines represent the four D channelsub-multiplex periods, and the fifth line represents the S channelsub-multiplex period. The sixth and seventh lines represent the period(equal to two sub-multiplex periods) which follow one multiplex 3transmission before the next multiplex 3 transmission. The handset 11listens for a reply in multiplex 2 during this period.

The five lines of the multiplex 3 transmission are each divided into thefour repetition periods. In the first four lines, each repetition periodcontains 36 D channel bits. Successive repetition periods of the samesub-multiplex are identical, but successive sub-multiplexes carrydifferent D channel information. Each repetition period of the fifthline in FIG. 8, representing the fifth sub-multiplex, contains 36 bitsof S channel. The S channel data in each repetition period is identical.Thus, the entirety of the data transmitted in a multiplex 3 burst iscontained in a single column in FIG. 8.

FIG. 9 shows in more detail the arrangement of data in a single columnof FIG. 8. In the D channel, only some of the bits carry useful data.Each 36 bit repetition period begins with 6 P bits forming a preambleportion of alternating logic "1"and "0". Then there are 10 data carryingbits, then a further 8 P bits, then a further 10 data carrying bits andfinally 2 further P bits. In this way, each repetition period contains20 data carrying bits of the D channel, divided into two stretches of 10bits separated by 8 bits of preamble. The data carrying bits can adoptany values, depending on the data being transmitted in the D channel,and therefore it is theoretically possible that the pattern of data bitsin the D channel may be identical to or may closely resemble thesynchronisation word of the S channel. If the D channel data wastransmitted continuously and this pattern of bits arose in it by chance,the base station 3 would mis-identify the received D channel repetitionperiod as belonging to the S channel, and therefore it would decodemultiplex 3 incorrectly. By splitting the data carrying bits of the Dchannel into sections of 10 bits, separated by 8 bits of preamble, the Dchannel in multiplex 3 can never contain a pattern of successive bitsresembling the pattern of the S channel synchronisation word.

As shown in FIG. 9, each repetition period of the S channelsub-multiplex of multiplex 3 begins with a preamble of 12 Pbits,alternating between logic "1"and logic "0". This is followed by 24W bits making the S channel sychronisation word. This arrangement of theS channel in multiplex 3 maximises the opportunity for the receivingbase station 3 to obtain bit synchronisation with the multiplex 3 signalbefore it receives the synchronisation word.

S Channel Structure

The structure of the S channel is very simple. It consists of 10 bits ofpreamble followed by 24 bits of synchronisation word in multiplex 2, asshown in FIG. 5, or 12 bits of preamble followed by a 24 bitsynchronisation word in multiplex 3, as shown in FIG. 9. The preamble isalways made up of alternating logic "1"and "0".

In the system of the preferred embodiment, there are four possible Schannel synchronisation words. Two of these are used only by basestations 3, and the other two are used only by handsets 11. When a partwishes to initiate a link, it uses an S channel synchronisation wordknown as a channel marker, abbreviated CHM. When the other part receivesthe signals, it replies using a normal synchronisation word, abbreviatedSYNC. When the link between the two parts is established, followingreception of this reply, the first part changes the synchronisation wordin the S channel in its transmissions from CHM to its version of SYNC.The version of CHM for the base station 3 is called the fixed partchannel marker, abbreviated CHMF, and the channel marker for the handset 11 is called the portable part channel marker, abbreviated CHMP.Similarly, the base station 3 version of SYNC is referred to as SYNCFand the handset 11 version of SYNC is referred to as SYNCP. CHMF andCHMP are bit inverses of each other, and SYNCF and SYNCP are bitinverses of each other.

Thus, if a base station 3 wishes to initiate a link it will transmit alink request using multiplex 2, with CHMF as the S channelsynchronisation word. A handset 11 receiving this transmission willreply in multiplex 2, using SYNCP as the S channel synchronisation word.Once the link is established the base station 3 will change itsmultiplex 2 bursts to use SYNCF as the S channel synchronisation wordinstead of CHMF.

If a handset 11 wishes to initiate a link, it will transmit a linkrequest in multiplex 3, using CHMP as the S channel synchronisationword. When this is detected by a base station 3, it will reply inmultiplex 2 using SYNCF as the S channel synchronisation word. Once thelink is established, the handset 11 will change its transmissions tomultiplex 2, using SYNCP as the synchronisation word.

When a base station 3 or a handset 11 is scanning channels to determinewhether another part is requesting a link with it, it will only react toa CHM synchronisation word, since this indicates that there is anotherpart wishing to set up a link. If a SYNC synchronisation word isdetected, this indicates that the channel contains a link which hasalready been set up, and therefore the part which is scanning channelsshould not react.

Base stations 3 are arranged to be capable of recognising CHMP andSYNCP, i.e. the S channel synchronisation words transmitted by a handset11, but are not capable of recognising CHMF or SYNCF. Therefore, a basestation 3 will never decode and respond to a multiplex 2 transmissionfrom another base station, even if it is received. Similarly, handsets11 can only recognise CHMF and SYNCF, and not CHMP and SYNCP, so thathandsets cannot recognise multiplex 2 and multiplex 3 transmissions fromeach other and therefore handsets can never initiate links directlybetween themselves but only with base stations.

Hardware

Following the above description of the manner in which the base station3 and the handset 11 exchange signals, the base station 3 and thehandset 11 themselves will now be described.

FIG. 10 shows an example of a handset 11. It has an aerial 25 fortransmitting and receiving signals in the radio link with the basestation 3. The aerial 25 may alternatively be provided out of sightinside the casing of the handset 11. The handset 11 has a microphone 27and a speaker 29 for use in telephone voice communication, and has akeypad 31 for controlling its operations. The bottom four rows of thekeypad provide a convention telephone keypad, allowing numbers to bedialled etc., including 0 to 9 numeric keys, a "hash" key and a "*" key.The keys 33 of the top row allow the user to control the radio link witha base station 3. By pressing an appropriate key 33 the user can accepta call to the handset 11 from a base station 3, or request access to thetelecommunications network 1 through a nearby base station 3.

The handset 11 may be provided with other conventional features, such asmemories for pre-storing telephone numbers, and a display. An example ofa handset 11 having a display 35 is shown in FIG. 11. Apart from thepresence of the display 35, this handset is the same as the handset ofFIG. 10.

FIG. 12 is a schematic diagram of the handset construction. The handsetis controlled by a control circuit 37, which is connected to the aerial25, the microphone 27, the speaker 29 and a keypad/display unit 39 whichprovides the keypad 31, and also the display 35 if it is provided. Oneor more batteries 41 provide power to the control circuit 37 and the keypad/display unit 39.

FIG. 13 shows an example of a base station 3. This has an aerial 43 forcommunication with cordless telephone handsets 11, and is also connectedthrough a conventional telephone connection 45 to the telecommunicationsnetwork 1 and though a power connection 47 to a conventional electricitypower source.

The base station 3 is also provided with a conventional wired telephonehandset 49, a display 51 and a keypad 53, which enable the base station3 to be used as a conventional telephone connected to thetelecommunications network 1 and also as an intercom station which cancommunicate with a cordless telephone handset 11 without involving thetelecommunications network 1. FIG. 14 provides a schematic view of theconstruction of the base station 3. It is controlled by a base stationcontrol circuit 55, which is connected to the wired handset 49, to thetelephone connection 45 and to a keypad/display unit 57 which providesthe keypad 53 and display 51 in a conventional manner. A power supplycircuit 59 receives electricity from the power connection 47, andsupplies power to the control circuit 55 and the key pad/display unit57. The power supply circuit 59 may include power storage means such asbatteries or capacitors, enabling the base station to operate for alimited period even when disconnected from an external electricitysupply.

The base station control circuit 55 includes a switching means 61,connected to the wired handset 49, the telephone connection 45 and theaerial 43. In one state of the switching means 61, it connects the wiredhandset 49 to the telephone connection 45 to permit normal telephoneoperation without the speech signals being processed by the controlcircuit 55 in the manner required for transmission over the radio link.In another state, the switching means 61 connects the wired handset 49and the aerial 43 to the remainder of the control circuit 55 so thatspeech signals pass between the wired handset 49 and the aerial 43having been processed as required to permit communication over the radiolink with a cordless handset 11. In a third state the switching means 61connects the telephone connection 45 to the remainder of the controlcircuit 55 in place of the wired handset 49, enabling the base station 3to act simply as a base for the radio link between the remote handset 11and the telecommunications network 1.

Where the functions of the wired handset 49 are not required, the basestation 3 may not include any of the wired handset 49, the display 51,the keypad 53 the keypad/display unit 57 and the switching means 61. Thebase station control circuit 55 would be permanently connected to boththe aerial 43 and the telephone connection 45.

As with the cordless handset 11, the base station aerial 43 may becontained within the casing of the base station 3.

FIG. 15 is a block diagram of the circuitry of the handset 11. Speechsounds received by the microphone 27 are converted into an electricalsignal which is provided to a speech encoder 63. The speech encoder 63includes an analogue-to-digital converter which converts the analogueelectrical signal from the microphone 27 into 8-bit digital signals witha sampling rate of 8 kHz. This results in a total bit rate of 64 kbitper second. The analogue-to-digital conversion is non-linear, and hasthe effect of performing Pulse Code Modulation (PCM) on the input.

The 8-bit data words are then compressed to 4-bit data words, therebyreducing the bit rate of the digital data to 32 kbit per second. Thecompression is done by Adaptive Differential Pulse Code Modulation(ADPCM). In this coding system, each 4-bit word represents the change invalue between the successive samples, rather than the absolute samplevalues themselves. This is an effective data compression technique forsignals which change relatively slowly, such as speech signals. The 32kbit per second data stream provides the contents of the B channel, andis provided to a B channel input to a programmable multiplexer 65 as4-bit parallel words at 8 kword per second.

The speech encoder 63 may also reverse the values of some bits in the Bchannel data, according to a predetermined pattern, in order to increasethe probability of changes of bit value between adjacent bits of thedata. This is for the benefit of the radio transmitting and receivingsystems, which may work better if the data value of transmitted andreceived signal bits changes frequently.

The programmable multiplexer 65 also receives D channel data and Schannel data at respective inputs. While the handset 11 is operating inmultiplex 1, the programmable multiplexer stores the continuouslyreceived 32 kbit per second data stream from the speech encoder 63. Theprogrammable multiplexer 65 outputs data in bursts, in accordance withthe burst mode operation of the radio link, at 72 kbit per second inaccordance with the data rate of the radio link. Thus, once in each 2 msburst period, the programmable multiplexer will output 64 bits of Bchannel data previously received from speech encoder 63 and stored, andwill sandwich the B channel data between 2 or 4 bits of D channel datato form the multiplex 1.2 or multiplex 1.4 data streams.

The data stream burst from the programmable multiplexer 65 is providedto a transmitter 67, which modulates the radio carrier frequency,received from a local oscillator 69, in accordance with the receiveddata stream. The resulting radio frequency burst is provided to theaerial 25 via a transmit/receive switch 71. The transmit/receive switch71 connects the transmitter 67 to the aerial 25 during the transmit partof each burst period and connects the aerial 25 to a radio receiver 73during the receive part of each burst period.

During the receive part of each burst period, the receiver 73demodulates the received signal from the aerial 25, using a carrierfrequency signal from the local oscillator 69. The demodulated 72 kbitper second data stream burst is provided by the receiver 73 to aprogrammable demultiplexer 75.

The programmable demultiplexer 75 allocates the received data bitsbetween the B channel, the S channel and the D channel in accordancewith the multiplex structure in which the handset 11 is currentlyoperating. When the handset is operating in multiplex 1, the 64 Bchannel bits received in each data burst are stored in the programmabledemultiplexer 75, and are then output to a speech decoder 77 as acontinuous stream of 4-bit parallel words at 8 kword per second.

The speech decoder 77 repeats the pattern of bit reversals applied tothe B channel data by the encoder in the base station 3, to obtain thecorrect data values, and then performs the inverse of the ADPCMalgorithm used to encode the speech data, so as to obtain 8-bit PulseCode Modulated words at a rate of 8 kword per second. The speech decoderthen converts this digital data to analogue data in a PCMdigital-to-analogue converter, and provides the output analogue signalto the speaker 29. The speaker 29 converts the analogue electric signalto sound to be heard by the user.

During multiplex 1 operation, the speech encoder 63 provides B channeldata to the programmable multiplexer 65 at 32 kbit per second. Thus ineach 2 ms burst period, the programmable multiplexer 65 receives 64 Bchannel bits. Since each multiplex 1 burst carries 64 B channel bits,the radio link carries the B channel at an effective average bit rateequal to the bit rate provided by the speech encoder 63. Similarly, theeffective average bit rate of received B channel data matches the bitrate of the continuous data transmission from the programmabledemultiplexer 75 to the speech decoder 77. Thus, there is an effectivecontinuous bidirectional B channel communication, in spite of the timedivision duplex burst mode nature of the radio link.

As will be well known to those skilled in the art, the speech encoder 63and the speech decoder 77 can conveniently be provided by a singlecircuitry unit known as a coder/decoder or codec.

The operation of the handset 11 is controlled by a system controller 79,and the timing of operations is controlled, in order to ensure burstsynchronisation, in response to signals from an S channel controller 81.The system controller 79 is typically a microprocessor-based ormicrocomputer-based control system, including a processor, a programmemory and a random access memory. The S channel controller 81 may beimplemented as a separate microprocessor or may be implemented insoftware for the same processor as the system controller 79. However, inview of the simple nature of the operations carried out by the S channelcontroller 81, and the need for high speed in its operations, it ispreferably implemented as dedicated hardware.

The system controller 79 sends control signals to the programmablemultiplexer 65 and the programmable demultiplexer 75, to instruct themwhich multiplex structure to adopt, and also to give them timing signalsso that they are properly synchronised with the radio link burststructure. The programmable multiplexer 65 and programmabledemultiplexer 75 may also send signals to the system controller 79 toinform it if a buffer used to store the data signals in the multiplexeror demultiplexer is approaching overflow or is empty.

Control signals from the system controller 79 control thetransmit/receive switch 71, so that it connects the transmitter 67 andthe receiver 73 to the aerial 25 alternately with the correct timing.

The system controller 79 selects the radio channel on which the handset11 is operating at any given moment, and instructs the local oscillator69 to generate a signal for the transmitter 67 and receiver 73 at theappropriate frequency. In a system intended for use in Great Britain inaccordance with the regulations issued by the Department of Trade andIndustry, the handset 11 may operate on any one of forty channels havingcarrier frequencies at 100 kHz spacings in the range 864.15 MHz to868.05 MHz. The system controller 79 will inform the local oscillator 69which channel has been selected, and the local oscillator 69 will informthe system controller 79 when its output signal has reached the selectedfrequency.

The system controller 79 may also send control signals to the speechencoder 63 and speech decoder 77 to mute the B channel at certain times.It is advantageous to mute the B channel during link set up and also ifit becomes necessary to re-establish a link during conversation, inorder to prevent the user from receiving unpleasant noises at thesetimes.

The system controller 79 also controls the D channel. It receivesincoming D channel data from the programmable D multiplexer 75 andprovides outgoing D channel data for transmission to the programmablemultiplexer 65. Some received D channel data is used purely to controlthe operation of the system controller 79, and some transmitted Dchannel data is generated within the system controller 79. Such dataincludes transmitted and received handshake signals and variousidentification signals which are exchanged between the handset 11 and abase station 3 during the establishment of a radio link. However, othertypes of transmitted D channel data will result from actions taken bythe user, and other types of received D channel data must be passed onto the user. For this reason, the system controller 79 also has acontrol signal connection with the keypad and display unit 39.

When a user is initiating a telephone call from the handset 11 thetelephone number to be dialled will be entered through the keypad 31.The key depressions will be notified by the keypad/display unit 39 tothe system controller 79, which will encode them for transmission in theD channel. In this way, the base station 3 is informed of the telephonenumber dialled by the user, and can transmit the appropriate diallingsignal to the telecommunications network 1.

If a base station 3 initiates a radio link with the handset 11, becausea telephone call has been received, the user must be alerted to thepresence of the incoming call. To do this, the system controller maycontrol a tone caller (not separately illustrated) to give an audiblenotification. Additionally, the system controller 79 may instruct thekeypad/display unit to provide a visual indication e.g. with a light. Ifthe user wishes to accept the call, this can be done by pressing a"line" key 33. This is notified by the keypad/display unit 39 to thesystem controller 79, which in turn notifies the base station 3 throughthe D channel.

If the handset 11 includes a display 35, information may be displayed onit in accordance with instructions from the system controller 79, bothbefore a user has accepted a call and during a conversation. The data tobe displayed will typically have been received by the system controller79 from the base station 3 over the D channel.

The S channel controller 81 receives S channel data from theprogrammable demultiplexer 75, and provides S channel data fortransmission to the programmable multiplexer 65. When the handset 11 isidle, and scanning radio channels to see whether a base station 3 iscalling it, the system controller 79 controls the transmit/receiveswitch 71 to connect the aerial 25 permanently to the receiver 73. Whena radio signal is received, bit synchronisation is achieved in theprogrammable demultiplexer 75, but the S channel controller 81 isresponsible for recognising the S channel synchronisation word andenabling burst synchronisation. Until burst synchronisation is achievedall received data is treated as potentially belonging to the S channeland is passed by the programmable demultiplexer 75 to the S channelcontroller 81. The S channel controller 81 searches the incoming datafor the S channel synchronisation word CHMF, which is used by a basestation 3 when it is wishing to set up a link.

When the S channel controller 81 recognises CHMF, it notifies the systemcontroller 79 that the base station channel marker has been received andalso provides a frame clock synchronised to the timing of the receivedburst. The system controller 79 uses the burst timing information fromthe S channel controller 81 to control the timing of the operation ofthe programmable demultiplexer 75, so that further transmissions fromthe base station 3 are decoded into the correct logical channels. Atthis stage, the programmable demultiplexer 75 will be operating inmultiplex 2. The programmable demultiplexer 75 divides the received databetween the S and D channels according to the multiplex 2 datastructure. Provided that the S channel data sent to the S channelcontroller 81 continues to include the synchronisation word CHMF, the Schannel controller will continue to confirm the burst synchronisation tothe system controller 79.

The system controller 79 decodes the data received on the D channel. Ifthis leads it to reply to the received transmission, it will instructthe programmable multiplexer 65 to begin operations in multiplex 2, withthe appropriate burst timing, and will control the transmit/receiveswitch 71 to alternate between connecting the aerial 25 to the receiver73 and the transmitter 67. At the same time, the system controller 79will instruct the S channel controller 81 to provide the SYNCPsynchronisation word to the programmable multiplexer 65 as S channelinput.

If the user of the handset 11 wishes to initiate a call, and thereforepresses one of the keys 33 on the keypad 31, the keypad/display unit 39will notify this to the system controller 79. The system controller 79searches through the RF channels, by changing the frequency of the localoscillator 69, until an empty channel is found. An empty channel isdefined as one on which the received radio frequency energy is below athreshold value. If the received radio frequency energy is above thethreshold value on all channels, the channel on which the least radiofrequency energy is received is defined as an empty channel.

The system controller 79 then instructs the programmable multiplexer 65to operate in multiplex 3, and instructs the S channel controller 81 toprovide the portable part channel marker CHMP to the programmablemultiplexer 65 as the S channel synchronisation word. Thetransmit/receive switch 71 is controlled to connect the aerial 25 to thetransmitter 67 and the receiver 63 in the pattern required for multiplex3 operation, and the system controller 79 ensures that the switching ofthe transmit/receive switch 71 is synchronised with the multiplex 3operation of the programmable multiplexer 65.

During receive periods, the programmable demultiplexer 75 passes anyreceived data to the S channel controller 81. The received data shouldinclude SYNCF. When this synchronisation word is identified, the Schannel controller 81 provides the system controller 79 with the bursttiming of the received signals. The system controller 79 then instructsthe programmable demultiplexer 75 to decode received data as multiplex2, in accordance with the received burst timing. Once the receivedchannel data has been decoded by the system controller 79, it willinstruct the programmable multiplexer 65 to switch to multiplex 2 withtiming synchronised with the burst timing information from the S channelcontroller 81. It will also change the timing of the control signals tothe transmit/receive switch 71 appropriately, and will instruct the Schannel controller 81 to provide SYNCP to the programmable multiplexer65 in place of CHMP.

A modified handset 11 may be used to communicate digital data e.g. toand from a portable personal computer or computer terminal, rather thancommunicate speech. In this case, the microphone 27, speaker 29 andkeypad/display unit 39 are replaced by an interface to the computer orterminal, and modification of the speech encoder 63 and speech decoder77 may be required. In particular, the computer or terminal willnormally provide and receive digital data, so that theanalogue-to-digital converter of the speech encoder 63 and thedigital-to-analogue converter of the speech decoder 77 will not berequired. Additionally, computer data is not normally suitable for datacompression using Adaptive Differential Pulse Code Modulation.Therefore, the data coding and decoding operations of the speech encoder63 and speech decoder 77 may need to be modified. Alternatively, if thecomputer or terminal can be set to operate at the data rate of 32 kbitper second, the encoder and decoder can be omitted entirely.

FIG. 16 shows a schematic block diagram of a base station 3. This is asimple base station, not including a wired handset 49, a display 51 anda key pad 53.

As can be seen, the general construction of the base station controlcircuit 55 is similar to that of the handset control circuit 37. Theprogrammable multiplexer 85, the transmitter 87, the local oscillator89, the transmit/receive switch 91, the receiver 93 and the programmabledemultiplexer 95 are substantially identical with the correspondingparts in the transmitter 11. The S channel controller 101 of the basestation 3 is also similar to the S channel controller 81 of thetransmitter 11, except that the base station S channel controller 101 isdesigned to recognise CHMP and SYNCP in the incoming S channel data, andto provide CHMF and SYNCF to the programmable multiplexer fortransmission, instead of the other way round.

The operation of the system controller 99 is generally similar to theoperation of the system controller 79 of the transmitter 11, but thereare some differences. First, when the base station 3 is trying to set upa radio link with a handset 11, it transmits in multiplex 2 rather thanmultiplex 3, and so the instructions to the programmable multiplexer 85and the timing signals to the transmit/receive switch 91 in thesecircumstances is different.

Similarly, when the base station 3 is scanning the radio channels todetect whether a handset 11 is calling it, it expects the handset 11 tobe calling using multiplex 3. Accordingly, once the S channel controller101 has notified the system controller 99 that the handset channelmarker CHMP has been received, the system controller 99 will instructthe programmable demultiplexer 95 to treat incoming signals as havingthe data structure of multiplex 3. Once the base station 3 has sent areply to a received multiplex 3 signal, it expects the handset 11 tochange to multiplex 2, and therefore it will instruct the programmabledemultiplexer 95 accordingly at this time.

Since the burst timing of the handset 11 is slaved to the timing of thebase station 3, except during multiplex 3 transmissions, the timinginformation received by the system controller 99 from the S channelcontroller 101 is not used to control the timing of the operations ofthe programmable multiplexer 85. The timing of the programmablemultiplexer 85 and the transmit/receive switch 91 is determined by aninternal clock of the system controller 99. However the programmabledemultiplexer 95 is controlled in accordance with the received bursttiming, both to enable correct decoding of multiplex 3 transmissionsfrom a handset 11 and to compensate for the effect of RF transmissiondelays on transmissions from the handset 11. The system controller 99may also use synchronisation timing information from the S channelcontroller 101 as one way of determining that a communication link witha handset 11 has broken down through loss of burst synchronization.

A second area in which the operations of the system controller 99 in thebase station 3 are different from the operation of the system controller79 in the hand set 11 is in its processing of D channel data. Thesignalling data received by the base station 3 from thetelecommunications network 1 will be different from the signalling datainput to the handset 11 by a user, and there will be correspondingdifferences in the D channel data received by each part over the radiolink. Accordingly, the programming of the system controller 99 in thedetails of its handling of D channel data will be different.

Also, the actions taken by the base station 3 during link initiation aredifferent from the actions of the handset 11, as will be described indetail later, and so the respective system controllers 99, 79 will beprogrammed differently in this respect.

The base station control circuit 55 includes a line interface 103, towhich the telephone connection 45 is connected. The line interface 103replaces the microphone 27, the loud speaker 29 and the keyboard/displayunit 39 in the arrangement of the control circuit. Signalling dataoutput by the system controller 99, typically in response to received Dchannel data, is conditioned by the line interface 103 and placed on thetelephone connection 45. Signals received from the telecommunciationsnetwork 1 over the telephone connection 45 are similarly interpreted bythe line interface 103 and provided to the system controller 99 asrequired. The line interface 103 also receives the decoded B channeldata stream from the decoder 97 and places this on the telephoneconnection 45, and receives the speech or other communication signalsfrom the telephone connection 45 and provides these to the encoder 83.

The manner of operation of the line interface 103 will be chosen inaccordance with the nature of the telecommunications network 1 to whichthe base station is connected. In particular, if the base station 3 3 isconnected to a conventional PSTN, the line interface 103 will send andreceive analogue signals over the telephone connection 45, whereas ifthe base station 3 is connected to an ISDN, the line interface 103 willnormally be required to send and receive 64 kbit per second pulse modemodulated signals.

In order to allow the base station 3 to communicate with variousdifferent types of handset 11, the encoder 83 and decoder 97 are enabledto carry out various encoding and decoding operations. They may be ableto use a plurality of different adaptive differential pulse codemodulation algorithms. They may also be able to use a digital dataprocessing algorithm or to pass signals through unaltered to enable thebase station 3 to be usable with portable computer and computer terminaltype handsets 11 as mentioned above. During the link set up procedure,while the base station 3 and the handset 11 are communicating inmultiplex 2, the handset 11 can indicate through the D channel the typeof coding and decoding it requires, and the system controller 99 of thebase station 3 will then control the encoder 83 and decoder 97 tooperate accordingly once multiplex 1 transmissions have begun.

FIG. 17 illustrates in block form the programmable multiplexers 65, 85.B channel data is output by the encoder 63, 83 as 4 bit parallel wordsat a rate of 8 kword per second. The 4 bits of each word are received inparallel, and they are stored in a B channel elastic store 105 in theprogrammable multiplexer under the control of an 8 kHz read clock whichis synchronised with the operations of the encoder 63, 83.

The D channel data is provided in 8 bit parallel words by the systemcontroller 99. This data may be provided intermittently, and the averagerate of D channel data will vary depending on the multiplex datastructure being used. The D channel data is received by a D channelelastic store 107, and is clocked into the elastic store by a clocksignal provided by the system controller 99.

In a similar manner, the S channel data is provided to an S channelelastic store 109, and is clocked into it by a clock signal synchronisedwith the operation of the S channel controller 81, 101. It would bepossible to eliminate the S channel elastic store 109, and provide the Schannel data from the S channel controller 81, 101 to the programmablemultiplexer 65, 85 with the correct timing for it to be slotted into thedata burst. However, this would require the operations of the S channelcontroller to be precisely synchronised with the operations of theprogrammable multiplexer, and the data structure of the burst would bedisrupted if there was any variation in bit or burst synchronisationbetween them. The use of the S channel elastic store 109 enables theprogrammable multiplexer to ensure that the S channel data is placed inthe data burst with the correct timing, regardless of any slightdifferences in timing of the S channel controller. Additionally, sincethe contents of the S channel will typically be the same from one databurst to the next, the S channel data can be stored in the S channelelastic store 109 and read out repeatedly, and the S channel controlleronly needs to provide new S channel data to the programmable multiplexerif the S channel synchronisation word is being changed. The S channelpreamble may be stored permanently in the S channel elastic store 109.

The multiplexing operation of the programmable multiplexer is controlledby a multiplex controller 111. This receives signals from the systemcontroller 79, 99, informing it which multiplex structure is being used,and also giving it the correct burst timing. The multiplex controller111 may also receive a clock signal from the system controller, oralternatively it may have an internal clock generator, which issynchronised to the burst timing signal from the system controller.

Signals are read out from the B channel elastic store 105, the D channelelastic store 107 and the S channel elastic store 109 under the controlof the multiplex controller 111, and the multiplexing of the signalstakes place in a signal combiner 113. The signal combiner 113 receivesinput from each of the elastic stores 105, 107, 109, and it selects thesignal received at one of these inputs to be passed on to the outputunder the control of an input select signal provided to the signalcombiner 113 by the multiplex controller 111. Simultaneously, themultiplex controller 111 provides control signals to the elastic stores105, 107, 109, so that each elastic store reads out one or more bits ofits contents as a serial bit stream, when its input to the signalcombiner 113 is connected to the output. The multiplex controller 111provides a 72 kHz clock to each of the elastic stores 105, 107, 109, sothat the signals are read out from these stores at the correct bit ratefor the data burst being assembled by the programmable multiplexer.

The B channel elastic store 105 and the D channel elastic store 107provide control signals to the multiplex controller 111, indicating theamount of data currently stored in the stores. The multiplex controller111 notifies the system controller 79, 99, if either of these stores isabout to overflow or alternatively if either of these stores contains nodata when the multiplex structure being transmitted requires data to besent in the relevant channel.

FIG. 18 is a block diagram of the programmable demultiplexers 75, 95.The incoming demodulated signal from the receiver 73, 93, is providedfirst to a re-timing unit 115. This continuously monitors changes in thelevel of the signal provided by the receiver, in order to maintain bitsynchronisation of the demultiplexer with the received signal. Theincoming data signal is then passed to a signal separator 117, whiledata on the received signal bit timing is provided to a demultiplexcontroller 119. The demultiplex controller provides a data distributionsignal to the signal separator 117, which controls the manner in whichthe signal separator 117 distributes the data received at its inputbetween three outputs, one for each of the B channel, the S channel andthe D channel.

The demultiplex controller 119 receives control signals from the systemcontroller 79, 99, informing it which multiplex structure the incomingdata should be treated as having. If the handset 11 or base station 3 isscanning the radio channels looking for a signal indicating that anotherpart wishes to set up a link with it, the demultiplex controller 119will direct the signal separator 117 to pass all data received by theprogrammable demultiplexer to the S channel. The S channel data isprovided directly to the S channel controller 81, 101. No elastic storeis used for the S channel in the programmable demultiplexer, as delaysto the S channel data in such an elastic store could prevent the Schannel controller from detecting the burst synchronisation of thereceived signal correctly.

Once the burst synchronisation of the incoming signal has been detected,the system controller 81, 101 will instruct the demultiplex controller119 to treat incoming data as having a specified multiplex structure,and it will also provide the demultiplex controller 119 with burstsynchronisation timing. In accordance with the instructions from thesystem controller, the demultiplex controller 119 will control thesignal separator 117 to distribute the incoming data between the threechannels.

The B channel data is provided to a B channel elastic store 121 and theD channel data is provided to a D channel elastic store 123. In allcases, the received data will be in the form of a serial bit stream at72 kbit per second. The data is clocked into the elastic stores 121, 123in accordance with a 72 kHz clock signal provided to the stores by thedemultiplex controller 119. The received signal bit timing informationprovided by the re-timing unit 115 to the demultiplex controller 119 isused by the demultiplex controller 119 to ensure that the 72 kHz clockis correctly synchronised with the data received by the elastic stores121, 123.

The demultiplex controller 119 controls the operation of the elasticstores 121, 123 so that they only store data while data for that storeis being provided by the signal separator 117. The stores provide thedemultiplex controller 119 with information on how much data theycontain, and the demultiplex controller 119 warns the system controller79, 99 if either store is empty or is about to overflow.

B channel data is read out of the B channel elastic store 121 as 4 bitparallel words, which are passed to the decoder 77, 97. The 4 bit wordsare read out at 8 kword per second in accordance with an 8 kHz clockprovided to the B channel elastic store 121 and synchronised with theoperations of the decoder.

The D channel information is read out of the D channel elastic store 123as required by the system controller 79, 99, as 8 bit wide parallelwords. This operation is performed in accordance with a read clocksignal provided to the D channel elastic store 123 by the systemcontroller.

FIG. 19 is a schematic diagram of the system controller 79, 99. Thesystem controller comprises a microprocessor 125, having a clock device127 connected to it in the conventional manner. A bus 129 for address,data and control signals connects the microprocessor 125 to a randomaccess memory 131 and a read only memory 133. The random access memory131 provides working memory for the microprocessor 125, and the readonly memory 133 contains the program for the microprocessor 125.

It will normally be necessary for handsets 11, at least, to perform aregistration operation before use, in order to acquire a code word whichpermits access to a base station 3 or a group of base stations 3, or toone of a plurality of facilities offered by a base station 3. In orderto permit such a code word to be stored safely, and retained even ifpower is removed from the device (e.g. when changing the batteries in ahandset 11), it is preferred that an electrically alterable read onlymemory (EAROM) 134 is provided in which such a code word may be stored.In an alternative, either the random access memory 131 or the read onlymemory 133 is an EAROM, and no separate EAROM 134 is provided, but thiswill normally be a more expensive implementation.

Other parts of the device are connected to the system controller asshown in FIGS. 15 and 16. As will be well known to those skilled in themicroprocessor art, these other devices can either be connected asperipheral devices, or as memory mapped devices. Memory mapped devicesare connected directly to the bus 129. Peripheral devices are connectedto an input/output interface 135, which is in turn connected to the bus129.

While the handset 11 or the base station 3 is active but not connectedin a radio link, it will be scanning the radio channels to determinewhether another device is seeking to initiate a radio link. At the sametime, the system controller 79, 99 must respond if a user presses abutton requesting that a radio link is set up, in the case of a handset11, or a telephone ringing signal is received from thetelecommunications network 1, in the case of a base station 3. This maybe done by programming the system controller so that it polls thekeypad/display unit 39, in the case of a handset 11, or the lineinterface 103 in the case of a base station 3, to determine whether asignal has been received which the system controller must respond to.Alternatively, the keypad/display unit 39 and the link interface 103 maybe connected through control lines of the bus 129 to interrupt inputs tothe microprocessor 125, so that the channel scanning operation of thesystem controller 79, 99 is interrupted if a signal is receivedrequiring the device to initiate a link itself. These alternatives inthe construction and programming of microprocessor controlled deviceswill be well understood by those skilled in the art.

FIG. 20 is a schematic block diagram of the S channel controller 81,101. The S channel controller has a CHM synchronisation word recogniser137 and a SYNC synchronisation recogniser 139. These continuouslycompare the 24 most recently received bits of data provided to the Schannel controller from the programmable demultiplexer 75, 95 withstored representations of the synchronisation words, and providerespective "CHM recognised" and "SYNC recognised" signals whenever amatch between the S channel input and the stored synchronisation wordsis obtained. The recognisers 137, 139 may each be implemented by a 24bit serial input shift register, having parallel outputs to first inputsof respective bit recognisers, the second inputs of which are connectedto hard-wired representations of the bits of the synchronisation word.

For each synchronisation word, the version for the handset 11 is the bitinverse of the version for the base station 3, and the synchronisationword recognisers 137, 139 can be switched between recognising thehandset words and recognising the base station words by inverting one ofthe two inputs to each bit comparator, or by inverting the input to theshift register. Thus, the recognisers are built so that they canrecognise either the handset words or the base station words, and in usewhich word is recognised is determined by a signal on line 141, whichindicates whether the S channel controller has been mounted in a handset11 or a base station 3.

The S channel controller 81, 101 also includes a CHM synchronisationword generator 143 and a SYNC synchronisation word generator 145. Thesemay each be constructed by providing a parallel input, serial output 24bit shift register, with the parallel inputs hardwired to provide theappropriate synchronisation word. Each synchronisation word generator143, 145 is designed to be able to generate either the handset words orthe base station words, by inverting the inputs or the output of theshift register, and the signal on line 141 determines which words willbe generated in operation of the generators.

As has previously been explained, the S channel controller 81, 101 of ahandset 11 will recognise the base station words and generate thehandset words, while the S channel controller 81, 101 of a base station3 will recognise the handset words and generate the base station words.The outputs of the synchronisation word generators 143, 145 are combinedby an OR gate 147, and are provided as the S channel input to theprogrammable multiplexer 65, 85.

The "CHM recognised" signal and the "SYNC recognised" signal areprovided, when the respective synchronisation word is recognised,directly to the system controller 79, 99 on respective lines 149, 151,and are also provided to a frame timing controller 153. The frame timingcontroller 153 also receives from the system controller informationabout which multiplex structure the received data is assumed to have,and also information about the state of the radio link. By combining thetiming of the "CHM recognised" or "SYNC recognised" signal with theinformation about the multiplex structure, the frame timing controller153 can generate a frame clock signal which provides burst timinginformation to the system controller 79, 99. Additionally, when the linkstate information indicates that the programmable demultiplexer 75, 95has been instructed to demultiplex incoming data in accordance with themultiplex 2 or multiplex 3 data structure at an assumed burst timing,the frame timing controller also provides a frame lock signal 157 to thesystem controller 79, 99, indicating whether the timing of the receivedsynchronisation word is in accordance with the assumed burst timing. Theframe clock signal 155 and the frame lock signal 157 are used by thesystem controller 79, 99 for such things as controlling the burst timingof the programmable multiplexer 65, 85 and the programmabledemultiplexer 75, 95, as has already been explained.

The frame timing controller 153 also provides control signals to the CHMgenerator 143 and the SYNC generator 145, when either of these isrequired to output the respective S channel synchronisation word to theprogrammable multiplexer.

Link Initiation Procedure

FIG. 21 shows in flow diagram form the actions taken by the handset 11and the base station 3 when the base station 3 initiates a link with thehandset 11. FIG. 22 shows the pattern of data burst transmissions duringthis process. FIGS. 23 and 24 are corresponding flow diagrams and databurst sequence diagrams for the case when a handset 11 initiates a linkwith a base station 3.

In each of FIGS. 21 and 23, two flow diagrams are shown: one for theactions taken by the base station 3 and one for the actions taken by thehandset 11. The flow of actions from step to step is shown in thicklines, and the passage of radio signals between the devices in varioussteps is shown in thin lines.

While the handset is turned on, but is not participating in a link, itperforms a channel-scanning loop. In step H1 it selects the next channelto scan. In step H2 it transmits nothing on the selected channel, butconnects its aerial 25 continuously to the receiver 73. The programmabledemultiplexer 75 passes any input data to the S channel controller 81.If the S channel controller 81 fails to detect the fixed part channelmarker S channel synchronisation word CHMF within a predeterminedperiod, the handset 11 abandons the channel in step H3, and returns tostep H1 to select the next channel. If all channels are scanned in turnwithout CHMF being detected, the handset 11 may cease operations for aperiod to conserve battery power, before scanning the channels again.

While a base station 3 is also not participating in a link, it will beperforming a similar scanning operation, as is shown in FIG. 23. Thisscanning operation will be interrupted if the base station 3 receives asignal such as a telephone ringing signal on the telephone connection45, indicating that it is required to set up a link with a handset 11.In this case, the base station will scan the available radio channels instep B1 to find an empty channel.

The base station 3 will then begin to transmit signals using multiplex2, in step B2. Between multiplex 2 transmission bursts, the base station3 will connect its aerial 43 to its receiver 93, to detect replies fromhandsets 11 using the multiplex 2 data structure with the SYNCP Schannel synchronisation word.

In its multiplex 2 transmission in step B2, the base station 3 willtransmit D channel data in a predetermined D channel code word format. AD channel code word will take several multiplex 2 data bursts to betransmitted. The structure of data transmissions on the D channel willbe described later.

The D channel code word transmitted by the base station 3 includes a PIDfield in which a "portable part identification" code is placed by thebase station 3 identifying the specific handset 11 it wishes to contact.The D channel code word also contains a LID field, in which the basestation 3 places a "link identification" code. Various different linkidentification codes may be used under different circumstances. When thebase station 3 is attempting to set up a link, the code placed in theLID field will be a base identification code (BID), identifying the basestation 3.

Although the base station 3 will only set up a link with one handset 11at a time, it may send out a call in step B2 to a plurality of handsets,and then establish the link with any one of them. In the multiplex 2transmissions by the base station 3 in step B2, the D channelinformation which the base station 3 wishes to transmit is repeatedcontinuously. If the base station 3 wishes to direct its signals to morethan one handset 11, then it will change the PID code in successivetransmissions of the D channel data so as to call each of the handsetsin turn.

When the channel selected by the handset 11 in step H1 is the samechannel as was selected by the base station 3 in step B1, the handset 11will detect in step H2 the multiplex 2 transmissions by the base station3 in step B2. Therefore, the handset 11 will find a CHMF code, and willmove to H4. In this step, the handset 11 uses the received CHMF code toachieve burst synchronisation with the base station 3, and the systemcontroller 79 instructs the programmable demultiplexer 75 to treatreceived data as being in multiplex 2. Accordingly, the multiplex 2transmissions from the base station 3 are decoded and the D channel datais passed to the system controller 79.

The system controller 79 assembles the D channel code words beingtransmitted by the base station 3, and examines the PID and LID fields.If the system controller 79 does not detect its own PID code within atime-out period, then in step H5 the handset 11 will conclude that thereceived call from the base station 3 is not intended for it, and itwill return to step H1. The system controller 79 may also determine whenthe sequence of PID codes, transmitted by the base station 3, isrepeated. When this happens, the system controller 79 should havedecoded every PID code being transmitted, and so if its own PID code hasnot been detected by this time, the handset 11 may return to step H1from step H5 even if the time-out period has not expired.

If, in step H5, the handset 11 decides to respond to the call from thebase station 3, because it has recognised its own PID code, it moves tostep H6. In this step, it begins to transmit in multiplex 2 as well aslisten for multiplex 2 transmissions from the base station 3. Thehandset 11 will place the SYNCP synchronisation word in the S channel,since it is responding to the base station 3 rather than initiating itsown call. It will use the D channel to transmit a reply code word, inwhich it will place its own identification code in the PID field, andwill place in the LID field the same code as was received in that fieldin the transmissions from the base station 3.

To avoid interference between two or more handsets 11 transmitting onthe same channel simultaneously, in response to a series of call signalsfrom the base station 3 identifying several handsets 11, the handset 11will transmit its reply immediately after it has received a D channelmessage containing its own PID code, and not after receiving a D channelmessage containing another PID code.

If the base station 3 detects a reply to its transmissions in step B2,it will check that the received S channel synchronisation word is SYNCP,and decode the received D channel information to check that itrecognises the returned PID code, and that the returned LID code is thesame as it sent out. If the base station 3 does not receive asatisfactory reply within a predetermined period, it abandons itsattempt to establish a radio link on that channel in step B3. The basestation then passes to step B4, where it determines whether it has beentrying to establish this radio link for more than a time-out period. Ifthe time-out period has not expired, it will return to step B1, selectanother free channel, and make a fresh attempt to establish a radiolink. If the time-out period in step B4 has expired, the base station 3moves to step B5, and ceases all attempts to establish a link.

If the base station determines in step B3 that a satisfactory reply hasbeen received, it proceeds to step B6. It continues to transmit its callto the handset or handsets, using multiplex 2. If satisfactory repliesare received from all the hand sets which are being called, the basestation 3 replaces CHMF in the S channel with SYNCF, to avoid alertingany further handsets 11 unnecessarily.

Once a handset 11 has identified that a base station 3 is transmitting acall to it, it can take action either to accept the call or to declineit. It may decline it either in response to some action by the user orit may have been pre-set to decline calls, for example through afunction similar to the known "do not disturb" function on aconventional telephone. A call will not be accepted until the userpresses one of the link control keys 33 on the handset keypad 31.Accordingly, in step H7 the handset 11 determines whether any action isrequired. If no action is needed, it passes to step H8 at which itdetermines whether the base station 3 is still transmitting the call.The base station 3 may cease to transmit the call either by entering alink with another of the handsets 11, and changing the transmittedmultiplex 2 message accordingly, or by ceasing to transmit on thechannel because no handset 11 has accepted the call within a pre-setperiod. If the call is no longer being transmitted, the handset 11returns to step H1 and once again scans the channels for a fresh call toit.

If it is determined in step H7 that action is required, the handsetpasses to step H9 to determine what the required action is. If therequired action is to refuse the call, it passes to step H10.

In step H10 the handset 11 continues to transmit in multiplex 2, butchanges the code in the LID field to a special "link decline" code. Itcontinues to transmit its own identity code in the PID field of the Dchannel. If the base station 3 receives a "link decline" message in stepB6, it may remove the associated PID code from the list of PID codeswhich are being called in rotation by the base station 3. The handset 11remains in step H10, and transmits the "link decline" code in responseto detecting its PID until a time-out period of e.g. 1 second has passedwithout its own PID being received. This confirms to the handset 11 thatthe base station 3 has received the "link decline" message and hasstopped transmitting this PID code. The handset 11 then returns to stepH1, and resumes scanning channels for further messages indicating that abase station 3 is attempting to set up a link. Because the base station3 with which it was previously in communication is no longertransmitting the PID code of this particular handset 11, the hand setwill not respond again to the base station 3 even when it scans thechannel being used by the base station, as it will determine in step H5that its PID is not being transmitted.

If the handset determines in step H9 that the required action is toaccept the call, it passes to step H11. In this step, it continues totransmit in multiplex 2, sending the same PID and LID codes in the Dchannel as in step H6. However, instead of transmitting its normalhandshake code, it transmits a special handshake code indicating "linkrequest". It continues to decode the multiplex 2 transmissions from thebase station 3, in order to receive the reply from the base station 3 tothe link request.

In step B7, the base station 3 determines whether it has received a linkrequest message from any of the handsets 11 it has been calling. If nolink request is received within a pre-set period, the base station 3passes to step B5, and abandons the attempt to set up a link. If a linkrequest is received, the base station 3 moves to step B8.

Since the base station 3 is now entering a link, it will change the Schannel synchronisation word in its multiplex 2 transmissions from CHMFto SYNCF, if this has not already been done in step B6. It will transmita reply to the handset 11 in which it replaces its normal handshake codein the D channel with a "link grant" code. In the PID field of the Dchannel code word, the base station 3 will transmit the identificationcode for the handset 11 to which it is granting the link.

In the LID field, it will transmit a different link identity code fromthe code transmitted in steps B2 and B6. The new LID code is anarbitrarily chosen code which identifies this specific link between thebase station 3 and the handset 11. If it ever becomes necessary tore-establish the link, as described below, the handset 11 will transmitlink re-establishment messages using the new LID code. This enables thehandset transmissions under these circumstances to be identified as anattempt at link re-establishment, and distinguished from a call from thehandset to set up a new link. If the original base identification codewas used as the LID code throughout an established link, this wouldincrease the possibility that a link re-establishment message from ahandset 11 would be misinterpreted by a base station 3 as a call to setup a new link.

When the handset 11 in step H11 receives the link grant message from thebase station 3, it passes to step H12. It will stop transmitting thelink request, and will change the code transmitted in the LID field ofthe D channel to the new code sent by the base station 3.

Once the base station 3, in step B8, has received a transmission fromthe handset 3 returning the new LID code, it knows that the link grantmessage has been received. Accordingly, the base station 3 moves to stepB9. Once the handset 11 has reached step H12 and the base station 3 hasreached B9, the link between them is established, and they communicatewith each other in multiplex 2. Subsequently, the base station 3 willinstruct multiplex 1 communication to begin. In response, the handset 11moves to step H13. Once the base station 3 receives multiplex 1transmissions from the handset 11, it moves to step B10. B channeltransmission may now begin.

The multiplex 2 transmissions between the handset 11 and the basestation 3 include codes indicating whether each side can supportmultiplex 1.4, and following this exchange the two parts agree onwhether to use multiplex 1.2 or multiplex 1.4 before moving to steps H13and B10.

FIG. 22 shows schematically the exchange of signals between the handset11 and the base station 3 when the base station 3 sends out a call whichis accepted by the handset 11.

First, the base station 3 uses multiplex 2 to transmit a D channelmessage 159, sending a call to a first handset 11. Next, the basestation 3 sends out a D channel message using multiplex 2 providingfurther D channel information, which any receiving handset may use. Thismay include data to be displayed on the display 35 of the handset toprovide the user with information about the call, or may include a Dchannel instruction to the handsets to provide a call signal to the usercorresponding to the normal ringing of the telephone. Next, the basestation 3 sends out a D channel code word 163 using multiplex 2, callinga second handset. It then repeats the D channel message 161. The basestation continues to alternate between calling handsets and sending thegeneral D channel message 161, calling each handset of a group ofhandsets in turn.

At some point the first handset 11 receives these messages from the basestation 3. Following the next transmission of the D channel word 159calling the first handset, it replies by sending a D channel word 165.

The base station 3 continues to send out the D channel words 159, 163,calling all the handsets in turn, interleaved with the D channel message161, and the first handset continues to send its reply message 165 inresponse to receiving each call message 159 directed to it, until thehandset user indicates that the call should be accepted. Following thenext transmission of the D channel word 159 calling the first handset,the handset 11 sends a link request message 167. The base station 3replies with a link grant message 169, and the link is established.

The base station 3 and the handset 11 then exchange D channel words 171using multiplex 2, until the base station 3 instructs the change tomultiplex 1. They then exchange multiplex 1 transmissions 173, carryingthe B channel and the telephone conversation begins.

FIG. 23 is a flow diagram corresponding to FIG. 21, but showing theactions taken by a handset 11 and a base station 3 when a link is set upin response to a call made by the handset 11.

If a base station 3 is active but not participating in a link, it willscan the channels to discover whether any handset 11 is attempting tocall it. In step B21, it will select a channel, and then in step B22 itwill listen for any transmissions on the selected channel. In step B22the base station 23 will transmit nothing. However as described withreference to FIGS. 6 to 9, it might not listen on the selected channelcontinuously but might listen only during every other 1 ms period,synchronised to the burst timing of an associated base station 3.

During the listen periods in step B22, the programmable demultiplexer 95will pass all received data to the S channel controller 101, in order todetect any CHMP channel marker synchronisation word transmitted by ahandset 11. The base station 3 will only respond to the CHMPsynchronisation word, and not the SYNCP synchronisation word, sincereception of the SYNCP synchronisation word indicates transmission froma handset 11 which has already made contact with some other base station3.

If the base station 3 determines in step B23 that the channel markercode word CHMP has not been received within a predetermined period, itreturns to step B21, selects the next channel, and begins to listen onthat channel. Once the base station 3 has scanned all the channels, itmay turn off for a while to save power, but this is less important forthe base station 3 than for the handset 11, as the base station 3 willnormally be connected to mains electric power.

If the handset 11 is turned on but is not participating in any link, itwill be performing a similar channel-scanning loop, as already describedwith reference to FIG. 21. However, this operation is interrupted if theuser presses a key 33, indicating that a link to a base station shouldbe established. In this case, the handset scans the channels in stepH21, to select an empty channel.

In step H22, the handset 11 will begin transmitting on the channel ithas selected using multiplex 3. In between the multiplex 3transmissions, its programmable demultiplexer 75 will pass any receiveddata to the S channel controller 81, in order to recognise the SYNCFsynchronisation word which should be contained in any reply from a basestation 3.

In the D channel of its multiplex 3 transmissions, the handset 11 willsend a D channel code word having a PID field and a LID field. In thePID field it will place its own handset identification code. In the LIDfield, it may place one of a variety of codes, depending on the servicerequired by the user.

If the handset is being used as an extension of a domestic telephone oras a numbered extension of a private branch exchange, the handset 11will transmit a LID code indicating that it wishes to make contact withthe specific domestic telephone or private exchange system with which ithas been registered. If the handset 11 is being used with a public"telepoint" system (which is a system in which a user can make telephonecalls through any one of various base stations in various geographicallocations) the LID code may identify the telepoint company or systemwith which the handset is registered and through which the user wishesto make the telephone call.

In an environment where several competing telepoint systems are present,it is preferable to define one or more LID codes which the handset 11can transmit to make contact with any base station within range,regardless of the system to which it belongs, and further LID codeswhich the handset 11 can transmit in order to make contact only withbase stations of one specified system. A further, special, LID code maybe used to enable a handset 11 to make contact with a base station 3purely for the purposes of registration, so that the base station 3 mayreceive and store the PID code of the handset 11, enabling it to becalled by the base station 3 in call set up sequences of the typeillustrated in FIG. 21. The handset 11 may also acquire further LIDcodes from a base station 3 in such a registration radio link.

The various LID codes are stored in the system controller 79 of thehandset 11 together with its PID code. These codes may be placed in oneof the memories of the system controller 79 during manufacture of thehandset 11 or may be entered subsequently through the keypad 31 in aregistration process, or may be received in a registration radio link asmentioned above.

If the base station 3 determines in step B23 that the handset channelmarker CHMP has been received, it will pass to step B24. In this step,it will still not transmit, but its programmable demultiplexer 95 willbe instructed to decode received data using the multiplex 3 structure,having the burst timing derived from the received CHMP word.Accordingly, the D channel data transmitted by the handset 11 will nowbe passed to the system controller 99 of the base station 3, where itwill be decoded. The system controller 99 will examine the PID and LIDcodes, and decide on the basis of these whether to respond to thehandset 11.

If it is determined in step B25 that no PID and LID codes requiring aresponse have been received within a pre-set period, the base station 3will return to step B21, select a new channel, and begin listening forfurther transmissions from handsets wishing to set up a link.

If the base station 3 determines in step B25 that it should respond tothe handset 11, it will move to step B26, and begin to transmit inmultiplex 2 with SYNCF in the S channel. The base station 3 willtransmit a D channel data word containing the PID code received from thehandset 11, and an arbitrary LID code to identify the link being set up.The base station 3 will expect the handset 11 now to switch to multiplex2 transmissions, using the SYNCP S channel synchronisation word.

In the D channel code word transmitted in multiplex 3 by the handset 11in step H22, the normal handshake code will be replaced by the "linkrequest" code. In the D channel code word transmitted in multiplex 2 bythe base station 3 in step B26, in reply to the hand set 11, the normalhandshake code will be replaced by the "link grant" code.

In step H23, the handset 11 determines whether it has received the SYNCFsynchronisation word from a base station 3 within a pre-set period. Ifnot, it passes to step H24. In this step, it determines whether a timeout period has expired since the handset 11 first began to request thelink. If the time out period has not expired, the handset 11 returns tostep H21, selects another free channel, and attempts to establish thelink on that channel. If the time out period has expired, the handsetpasses to step H25, and abandons the attempt to set up the link.

If it is determined in step H23 that SYNCF has been received, thehandset 11 passes to step H26. In this step, it temporarily ceasestransmission, and decodes the received multiplex 2 transmissions fromthe base station 3, while using the received SYNCF synchronisation wordto achieve burst synchronisation with the transmissions from the basestation 3.

The handset 11 is now able to decode the D channel informationtransmitted by the base station 3. In step H27 it determines whether ithas received within a pre-set period a D channel code word containingits PID and the "link grant" code. If such a D channel code word is notreceived within the pre-set period, the handset 11 moves to step H25,and abandons the attempt to set up the link. If the handset 11 doesreceive a link grant message accompanied by its own PID, it moves tostep H28. In this step, it begins multiplex 2 transmissions, using SYNCPas the S channel synchronisation word. In its D channel message, it willcontinue to send its own PID code, but will change the LID code to thelink identification code received from the base station 3. In this step,the handset 11 will continue to listen for multiplex 2 transmissionsfrom the base station 3, and will maintain burst synchronisation withthe base station 3.

Once the base station 3 has received a multiplex 2 transmission from thehandset 11, using SYNCP as the S channel synchronisation word andreturning the LID code sent out by the base station 3, it knows that thelink grant message has been received.

The base station 3 now moves to step B27, in which it ceases to transmitthe link grant message, and exchanges D channel information with thehandset 11 using the multiplex 2 data structure. Once the hand set 11has reached step H28 and the base station 3 has reached step B27, theradio link has been established. Subsequently, the base station 3 willinstruct the beginning of multiplex 1 communication. The handset 11 willmove to step H29 and the base station 3 will move to step B28. B channelcommunication may now begin.

FIG. 24 shows schematically the pattern of signals transmitted when ahandset 11 successfully initiates a link with a base station 3. First,the handset 11 transmits a series of link request messages 175, inmultiplex 3. When the base station 3 receives these messages, anddecides to grant the link, it replies with link grant messages 177 inmultiplex 2. On receipt of the link grant messages 177, the handset 11ceases multiplex 3 transmission, synchronises its burst timing with thesignal from the base station 3, and begins to decode the receivedmultiplex 2 bursts. When it has decoded a link grant message 177, thehandset 11 begins transmitting messages 179 using multiplex 2. The twoparts continue to exchange messages 179 in multiplex 2, until the basestation 3 instructs the change to multiplex 1 messages 181.

As explained above, in some circumstances the handset 11 may transmit alink request message in multiplex 3 using a LID code identifying severalbase stations 3, any of which the handset 11 can establish the linkwith. If more than one such base station 3 is within range of thehandset 11, the base station 3 which first scans the channel on whichthe handset 11 is transmitting will normally be the first to grant thelink, and the link will successfully be set up with that base station.However, two base stations 3 may, by chance, transmit link grantmessages to a handset 11 simultaneously. In this case, the handset 11will most probably fail to decode either message successfully.Accordingly, in step H23 the handset 11 will determine that it has notreceived SYNCF, and will pass through step H24 to step H21. It willselect another empty channel and repeat its multiplex 3 transmissions onthat channel. Since the handset 11 will not reply to the link grantmessages from the base stations 3 in this case, both base stations willconclude that the link has failed, and will return to step B21. Eachbase station will select a new channel and begin listening for thetransmission of CHMP from a handset 11.

In this case, if both base stations 2 select the next channel, they willcontinue to scan each channel at the same time as each other, andattempts to grant links to handsets 11 will continue to fail for thesame reason. In order to prevent this, the channel selected in step B21under these circumstances is not the next channel. Instead, the basestations 3 follow rules designed to reduce the probability that theywill continue to scan the same channel at the same time. This may bedone by providing a channel selection algorithm which operates randomly,so that the channel selected in step B21 under the circumstances ischosen randomly. Alternatively, each base station 3 may be programmed toreturn to a particular channel under these circumstances, and nearbystations are programmed to return to different channels. The randomchannel selection algorithm is preferred. If the base stations areprogrammed to return to a specific channel, there is the possibilitythat incorrect programming may direct two nearby base stations to returnto the same channel each time, in which case conflict between them willnot be resolved.

D Channel Structure

As mentioned above, messages in the D channel are transmitted using codewords. Each code word is 64 bits long. A string of code words may besent in succession, as a D channel data packet. In this case, the firstcode word must have a first particular format, and is known as anaddress code word (ACW), and the remaining code words of the packet musthave a different specified format and are known as data code words(DCW). In a packet, an address code word may be followed by up to fivedata code words. When only one code word is sent, not followed by anyothers, it must be an address code word.

The rate at which D channel bits are sent is determined by the multiplexdata structure being used in the radio link. It will always take severalbursts to send a single D channel code word. The maximum speed isprovided by multiplex 2, in which 32 bits of the D channel are sent ineach burst, so that two bursts are required for a code word. The slowesttransmission of D channel is with multiplex 1.2, in which only two bitsof D channel are sent per burst. In this case, 32 bursts are required tocarry a code word.

If at any time, there is no D channel information to be sent, themultiplex structures nevertheless require D channel bits to betransmitted. In this case, a signal called "IDLE D" may be sent to fillthe D channel. When IDLE D is being sent, the D channel bits alternatebetween 1 and 0.

In order to alert the receiving part that useful D channel informationis about to be transmitted, every address code word is preceded by astandard 16 bit D channel synchronisation pattern called SYNC D. As wellas informing the receiving part that an address code word follows, theSYNC D pattern ensures that the D channel decoding operation of thesystem controller 79, 99 is synchronised with the boundaries of the codewords, so that each code word is decoded correctly.

Thus, a typical sequence of D channel transmissions, assembled from aplurality of data bursts, might be as shown in FIG. 25. A period duringwhich IDLE D is transmitted ends with the transmission of the 16 bitSYNC D pattern. This is immediately followed by a 64 bit address codeword, and then one or more 64 bit data code words.

In multiplex 1, the first bit of the SYNC D pattern must always betransmitted as the first bit of a multiplex 1 burst. In multiplex 2, the16 bit SYNC D pattern must always be transmitted as the final 16 bits ofa burst, and in multiplex 3 the first bit of the 16 bit SYNC D patternmust always be the first bit of useful D channel information transmittedafter the initial 6 bit D channel preamble in each repetition period ofthe first submultiplex. When multiplex 2 or 3 is being used, this placesthe SYNC D pattern as soon as possible after the S channelsynchronisation word, and therefore maximises the likelihood that theSYNC D pattern will be detected promptly following burstsynchronisation.

FIG. 26 shows the structure of a message in the D channel. An addresscode word 183, optionally followed by up to five data code words 185transmitted in a continuous string, as shown in the first line of FIG.26, forms a D channel packet 187, shown in the second line of FIG. 26.Several packets 187 may be combined to create a D channel message of anylength, as shown in the bottom line of FIG. 26.

In order to maintain at least a minimum rate at which handshake signalsare exchanged, successive packets of a D channel message are notnecessarily transmitted immediately one after another. Instead, aspecial address code word, not followed by any data code words, may betransmitted between successive packets of a message. The special addresscode word carries handshake and identification signals.

FIG. 27 shows the general format of a D channel code word. The code wordis made up of eight octets, each illustrated as one line in FIG. 27, andeach octet is in turn made up of eight data bits.

When the code word is transmitted over the D channel, bit 1 of octet 1is transmitted first. This is the top right hand bit in FIG. 27. Next,bit 2 of octet 2 is transmitted. This is the bit second from the rightin the top line of FIG. 27. The remaining bits of octet 1 are thentransmitted in order. Then octet 2 is transmitted in order from bit 1 tobit 8. The remaining octets are transmitted in order in the same manner,so that the last bit of the code word to be transmitted is bit 8 ofoctet 8, which is the bottom left hand bit in FIG. 27.

Bit 1 of octet 1 is used to indicate the type of code word. For anaddress code word, the bit is set to "1". For a data code word, the bitis set to "0". Bit 2 of octet 1 determines the code word format. Anaddress code word can either be in fixed format or variable lengthformat. A fixed format address code word is used for transmittinghandshake and identity messages, and will be described in more detailwith reference to FIG. 28. A fixed format address code word is notfollowed by any data code words. In an address code word, bit 2 of octet1 is set to "0" to define a fixed format address code word. Bit 2 ofoctet 1 is set to "1" to indicate a variable length format code word.Variable length format indicates that the length of the packet may vary,i.e. data code words may be present. Data code words are always invariable length format, and therefore this bit should always be set to 1for a data code word. All normal D channel messages are carried byvariable length format code words. A variable length format address codeword may be followed by up to five data code words, but may also form apacket without being followed by any data code words.

The significance of the remaining bits of octet 1, and all bits ofoctets 2 to 6, depends on whether the code word is a fixed formataddress code word, a variable format address code word or a data codeword.

Octets 7 and 8 always carry a check code. The first fifteen bits of thecheck code, from bit 1 of octet 7 to bit 7 of octet 8, provide a cyclicredundancy check (CRC) code. Such codes, and methods of generating them,are well known. Bit 8 of octet 8 is a parity bit, which is chosen so asto give the whole 64 bit code word even parity.

The structure of a fixed format address code word is shown in FIG. 28.Bit 1 of octet 1 is set to "1" to indicate that it is an address codeword, and bit 2 of octet 1 is set to "0" to indicate that it is a fixedformat word. Bits 3 and 4 of octet 1 carry the hand shake code. Bit 5 ofoctet 1 encodes the multiplex 1 signalling rate. It is set to "1" toindicate multiplex 1.4, and is set to "0" to indicate multiplex 1.2.

The remainder of octet 1, and octets 2, 3 and 4 carry the PID code.Preferably, this is divided into two sections. Octet 4 alone carries amanufacturer identity code, which can be allocated to a manufacturer bya regulatory authority. The remainder of the PID code is allocated bythe manufacturer, and indicates one specific handset 11 produced by themanufacturer. Octets 5 and 6 carry the LID code, and octets 7 and 8carry the cyclic redundancy check and the parity bit.

The fixed format address code word is transmitted during link set up, tocarry the PID and LID codes and the "link request" and "link grant"messages, as described with reference to FIGS. 21 to 24. "Link request"is transmitted by setting the handshake bits 4 and 3 of octet 1 to "00"."Link grant" is transmitted by setting the handshake bits 4 and 3 ofoctet 1 to "01".

When the handset 11 transmits "link request", it sets the signallingrate bit (bit 5 of octet 1) to "1" if the handset can support multiplex1.4, and to "0" if it can only support multiplex 1.2. When the basestation 3 sends the "link grant" message, it will set bit 5 of octet 1to "1" only if the base station 3 can support multiplex 1.4 andadditionally it has received a bit "1" at this position from the handset11, indicating tat the handset can also support multiplex 1.4. If eitherdevice can only support multiplex 1.2, the base station 3 sets bit 5 ofoctet 1 to "0" in the "link grant" message, informing the handset 11that multiplex 1 transmission will take place using multiplex 1.2. Thisconcludes the "negotiation" operation between the two devices concerningwhich version of multiplex 1 to use.

FIG. 29 shows the structure of a variable format address code word. Inthis code word, bit 1 of octet 1 is set to "1" to indicate that it is anaddress code word, and bit 2 of octet 1 is set to "1" to indicate thatit is in variable format. The significance of bits 3, 4 and 5 of octet 1depends of bit 6 of octet 1. If further code words follow in the Dchannel packet, bit 6 of octet 1 is set to "0". In this case, bits 3, 4and 5 give in binary the number of further D channel code words in thepacket following the code word in which these bits appear. Thus, if thepacket contains three code words in total, bits 3, 4 and 5 of octet 1 ofthe address code word (which will be the first code word of the packet)will be set to "2" or "010" in binary, to indicate that two further codewords follow.

If bit 6 of octet 1 is set to "1", this indicates that the code word isthe final code word of the packet. In this case, it is possible thatonly some of the data carrying octets of the code word carry usefuldata. Therefore, in this case bits 3, 4 and 5 of octet 1 give the numberof data carrying octets of the code word which carry useful data. Whenthe code word is interpreted by the receiving system controller 79, 99,it will use this information to ignore any remaining octets, which donot carry useful data, in the final code word of a packet.

Bit 7 of octet 1 is set to 1 to indicate that the current packet isfollowed by further packets of D channel information, and is set to "0"for the last packet of a D channel message.

Bit 8 of octet 1 is set to "0" to indicate that octet 2 has its normalsignificance as a control message. In the illustrated embodiment, thisis always the value of this bit, but it provides the facility ofredefining the meaning of octet 2 of a variable format address codeword, if this is desired. Through this redefinition of the meaning ofoctet 2, this bit enables the interpretation of the entire packet to bechanged.

Octet 2 of the variable format control word is a control octet. Bit 3 ofoctet 2 is set to "1" to indicate that the receiving device mustacknowledge successful reception of the D channel packet. In this case,bit 4 of octet 2 will be the packet number, and alternates between "0"and "1" for successive packets. Bit 2 of octet 2 is used to acknowledgereceived D channel packets from the device at the other end of the link,when this is required. This bit is set to the expected value of bit 4 ofoctet 2 of the address code word for the next packet to be received fromthe other device. When bit 3 of octet 2 is set to "0", acknowledgementof the packets is not required, and bit 4 of octet 2 has nosignificance. Whether bit 2 of octet 2 has significance will depend onwhether the device at the other end of the radio link has requestedacknowledgement of its packets.

Bit 3 of octet 2 must be set to "1", requiring that packets areacknowledged, whenever a D channel message contains more than 1 packet.

Bit 1 of octet 2 is set to "0" if the last D channel packet received bythe transmitting device was accepted. If a received D channel packet isrejected, e.g. because the CRC check fails for one of the code words,bit 1 of octet 2 in the next transmitted variable format address codeword is set to "1", and bit 2 of octet 2 is set to the value of bit 4 ofoctet 2 in the address code word of the received, but rejected, packet.

Bit 5 of octet 2 specifies whether the D channel packet is "informationtype" or "supervisory type". The contents of "supervisory type" packets(bit 5 is set to "0") relate to operations controlling and maintainingthe radio link. Such packets may include instructions asking the otherdevice to increase or decrease the power at which it is transmitting, tore-establish the link on the same channel, or to re-establish the linkon another, specified, channel. Another supervisory message is theFILL-IN message, which serves a special purpose which will be describedlater.

All other D channel messages are carried by "information type" packets(bit 5 is set to "1"). These will include messages sent by a basestation 3 to instruct a handset 11 to emit a ringing tone to alert auser of an incoming call, or to send messages to be displayed on thedisplay of the handset 11. "Information type" messages sent by a handset11 to a base station 3 typically inform the base station 3 that certainkeys of the keypad 31 have been pressed. The messages for changingmultiplex structure between multiplex 1 and multiplex 2 are also carriedby "information type" packets.

D channel messages are only permitted to be constructed from more thanone packet, as shown in the bottom line of FIG. 26, if the packets are"information type" packets. "Supervisory type" packets must each beindependent, with bit 7 of octet 1 of the address code word set to "0".

Octets 3, 4, 5 and 6 of the address code word carry the D channelmessage contents. Octets 7 and 8 carry the CRC code and the parity bit.

FIG. 30 shows the structure of a data code word. Bit 1 of octet 1 is setto "0", to indicate that it is a data code word, and bit 2 of octet 1 isset to "1", as data code words are only permitted in variable format.Bits 3, 4, 5 and 6 of octet 1 have the same meaning as for the variableformat address code word shown in FIG. 29. Bits 7 and 8 of octet 1 haveno significance, and are set to "0".

The data code word does not include a control octet, and accordingly theD channel message contents are carried by octet 2, octet 3, octet 4,octet 5 and octet 6. Octets 7 and 8 carry the CRC code and the paritybit.

In "information type" packets, the D channel messages in the messagecontent portions of the code words are provided in "identifier, length,contents" format, in a fashion similar to that already known for ISDNdata. In this format, bit 8 of the first octet of the message is set to"1" to indicate a fixed length message, and is set to "0" to indicate avariable length message. A fixed length message consists of only oneoctet. Unless it is known that a packet continues a message started in aprevious packet, it is always assumed that the first message contentsoctet of the address code word in an "information type" packet (i.e.octet 3 of the address code word) is the first octet of a D channelmessage.

FIG. 31 shows the format of a fixed length message. Bit 8 is set to "1"to identify that it is a fixed length message. Bits 7, 6 and 5 provide acode identifying the type of message being carried. Since the message isin fixed length format, no length information is required, and bits 4,3, 2 and 1 provide the message contents.

This message format is used only to carry very simple messages, such asmessages controlling the manner in which variable length format messagesshould be interpreted.

FIG. 32 shows the format of a variable length D channel message. Thisconsists of at least three octets, and maybe more.

In the variable length format, bit 8 of the first octet is set to "0",to indicate that it is a variable length format message. The other sevenbits of the first octet provides the identifier code, identifying thetype of message being sent. The second octet is the length code. This isthe number of remaining octets in the message, following this lengthcode octet. Thus, if the total message is four octets long, anidentifier and format type octet, a length code octet and two furtheroctets, the length code octet will indicate that two further octetsfollow. All remaining octets of the variable length message carry themessage contents.

Handshake Codes and Link Re-establishment

As has already been described, the fixed format address code word ofFIG. 28 is used during link set up to carry the "link request" and "linkgrant" messages, to carry the negotiations for selecting betweenmultiplex 1.2 and multiplex 1.4, and to carry the PID and LID codes.After the link has been established, this D channel code word is alsotransmitted from time to time to carry handshake signals. The PID andLID codes can be used during the link to confirm that the link continuesto be established between the same two devices.

In order to maintain continuity of the link, handshake words must beexchanged with at least a certain minimum frequency. The separation of Dchannel messages into packets allows this to be done. Between successivepackets of the same message, the fixed format address code word can betransmitted, to maintain the handshake rate. This does not disrupt thetransmission of D channel messages, as every variable format addresscode word indicates whether further packets (and therefore furthervariable format address code words) will follow in the same message. Thefixed format address code word will be recognised as not being a packetof the message, and the assembly of the D channel message will resumewhen the next variable format address code word is received.

In the fixed format address code word, bits 3 and 4 of octet 1 carry thehandshake message. Therefore, four handshake messages are possible. "00"means "link request". "01" means "link grant". "10" means "ID OK". "11"means "ID LOST". The use of "link request" and "link grant" during linkset up has already been described with reference to FIGS. 21 to 24.These handshake messages are only sent for the purposes described. Atall other times, the normal handshake message in the fixed formataddress code word is "ID OK". This code serves as a handshake code, andalso confirms to the receiving device that the transmitting device hasreceived a handshake code from the receiving device within a pre-setperiod. The "ID LOST" code also serves as a handshake code, butindicates to the receiving device that the transmitting device has notreceived a valid handshake code from the transmitting device within thepre-set period. The use of "ID LOST" enables failure of a link to bedetermined promptly, so that it can be re-established with the minimumdelay.

When two parts, a handset 11 and a base station 3, are connected in aradio link, each part will transmit a handshake code, using the fixedformat address code word, at a rate not greater than once every 400 ms,and not less than once every second. The timing of the transmission of ahandshake code word is not dependent on the timing of the reception of ahandshake code word from the other part. If either part determines thatit has not received a valid code word for more than 1 second, itconcludes that handshake has been lost. If either side has not receiveda valid code word for at least 3 seconds, the parts are permitted tore-establish the link on another channel. However, if either part hasnot received a valid handshake code for 10 seconds, attempts tore-establish the link must cease, and the link must be treated as havingbeen terminated.

The prohibition on re-establishing on another channel after less than 3seconds prevents undesirable rapid channel switching, which mightinterfere with the operation of other devices trying to use otherchannels, and prevents unnecessary channel switching in response to abrief burst of radio frequency noise or interference. The requirement toclose down the link 10 seconds after the loss of handshake preventsattempts to re-establish the link from continuing indefinitely.

If two parts are exchanging handshake signals very rapidly, which wouldbe possible using multiplex 2, there is a chance that each side couldreceive a valid handshake signal once every 3 seconds, even though thequality of the link was very poor. Under these circumstances, the partswould be forbidden to change channel regardless of the poor quality ofthe link. Therefore, handshake signals are not transmitted morefrequently than once every 400 ms, even if there is spare D channelcapacity to carry more frequent handshake codes.

Every time a part transmits a handshake code, it resets a transmittimer. Every time it receives any of the four possible handshake codes,it resets a receive timer. If the received handshake code is "ID OK", italso resets a link timer, but the link timer is not reset if thereceived handshake code is any of the other handshake codes.

When the transmit timer indicates that 400 ms have passed since the lasttime the part transmitted a handshake code, it prepares to transmit ahandshake code as soon as the structure of data being transmitted on theD channel permits. Just before it transmits its handshake code, itchecks the receive timer. Provided that the receive timer indicates thatone of the handshake code words has been received within the pastsecond, the part transmits the "ID OK" handshake code. Otherwise, ittransmits the "ID LOST" handshake code. If the part subsequentlyreceives any valid handshake code, it will reset its receive timer, andreturn to sending "ID OK" handshake codes.

If no handshake codes are received, or if only handshake codes otherthan "ID OK" are received, the link timer is not reset. Once the linktimer indicates that 3 seconds have passed since the last "ID OK" wasreceived, the part will automatically begin link re-establishment. Ifthe part is a handset 11, it will begin to transmit in multiplex 3, andif the part is a base station 3 it will begin listening for the CHMP Schannel synchronisation word transmitted in multiplex 3 by the handset11. Link re-establishment follows the same procedure as the proceduredescribed with reference to FIGS. 23 and 24 used to set up a link whenthe call is initiated by the handset, except that in the multiplex 3transmissions by the handset, the code transmitted in the LID field ofthe D channel is the most recent link identification code which was usedby the parts in the link which they are trying to re-establish, and notthe code normally used by the handset to establish a new link.

When the link is re-established, reception of the "ID OK" handshake codewill reset the link timer. If the link timer shows that this code hasnot been received within 10 seconds of the last time it was received,the part will abandon attempts to re-establish the link.

Because each part does not reset its link timer when the "ID LOST"handshake code is received, the link timers of the two parts will alwaysshow times within 1 second of each other, and normally less than 500 msof each other. This ensures that the two parts begin to attempt linkre-establishment, and if necessary abandon attempts at linkre-establishment, at almost the same time, even if the nature of theproblem with the link is such that signals in one direction continue tobe received successfully while signals in the other direction are not.

The effect of the "ID LOST" handshake code on the link timers, when alink breaks down in one direction only, is illustrated in FIGS. 33 and34. FIG. 33 illustrates the case where signals from the handset 11 failto reach the base station 3, although signals from the base station 3continue to reach the handset 11.

Initially in FIG. 33, the quality of the link is good and "ID OK"handshake codes are transmitted by both parts. However, interferencethen prevents the base station 3 from receiving handshake codes from thehandset 11, and the last handshake code received by the base station 3occurs at time A. At time B, the base station 3 transmits its nexthandshake code. As this is less than 1 second from time A, it transmits"ID OK". However, next time it transmits a handshake code, it transmits"ID LOST", as it is now more than 1 second since time A. Additionally,since no further handshake codes are received by the base station 3, itslink timer is not reset after time A.

Since the handset 11 is continuing to receive valid handshake codes fromthe base station 3 at less than 1 second intervals, it continues totransmit "ID OK" as its handshake code. However, since it is receiving"ID LOST" instead of "ID OK", the handset 11 does not reset its linktimer after time B.

At time C, which is 3 seconds from time A, the base station 3 preparesfor link re-establishment. It stops transmitting over the radiofrequency channel, and begins to scan for multiplex 3 transmissions fromthe handset 11. At time D, which is 3 seconds after time B, the handset11 stops its previous transmissions over the link, and beginstransmitting in multiplex 3, over the same or different channel, toinitiate link re-establishment.

The period between times C and D is the same as the period between timesA and B. Since "ID OK" was transmitted by the base station 3 at time Bbecause this time was less than 1 second after time A, it can beguaranteed that these two times are less than 1 second apart. Typically,they will be less than half a second apart. Therefore times C and D,when the respective parts move to link re-establishment, will besimilarly close together.

FIG. 34 shows the case where, due to interference, the handset 11 ceasesto receive handshake signals from the base station 3, but the basestation 3 continues to receive handshake signals from the handset 11.Initially, the link quality is good, and the parts both transmit "ID OK"handshake codes. Subsequently, the handset 11 ceases to receive thehandshake codes transmitted by the base station 3, and the lasthandshake code received by the handset 11 is sent at time E. From thistime onwards, the handset 11 receives no handshake codes at all, and soit does not reset either its receive or its link timers.

The next time the handset 11 sends a handshake code, it is still lessthan 1 second since it received the "ID OK" code at time E. Thereforethe base station 11 transmits an "ID OK" code, at time F. However, whenthe handset 11 comes again to transmit a handshake code, it is more than1 second after time E, and accordingly it transmits "ID LOST".

Since the base station 3 continues to receive handshake codes from thehandset 11, it continues to reset its receive timer and transmits "IDOK".

However, it is now receiving "ID LOST" signals. The last "ID OK" signalreceived by the base station 3 is sent at time F, and this is the lasttime that the link timer at the base station 3 is reset.

At time G, the handset 11 is informed by its link timer that it is 3seconds from time E. The handset 11 therefore stops its previoustransmission of signals over the radio link, and begins to try tore-establish a link by transmitting in multiplex 3. Shortly afterwards,at time H, the link timer at the base station 3 informs the base stationthat it is 3 seconds after time F, and the base station 3 also moves tolink re-establishment, and begins to scan for the multiplex 3transmissions by the handset 11.

Since times E and F must be less than 1 second apart, times G and H arealso less than 1 second apart.

Regardless of the direction in which the link breaks down, it is alwaysre-established from the handset 11 to the base station 3, and not viceversa. In some situations, the link may be established between thehandset 11 and any one of several base stations 3 at differentlocations. This may be the case if the base stations 3 are part of apublic telepoint system, and may also be the case if the base stations 3are all connected to the same private branch exchange, e.g. for a largeindustrial site where base stations at various different locations arerequired to cover the whole area of the site.

In one of these situations, the base stations 3 will all be connected toa central controller, e.g. a computer, and a link may fail because ahandset 11 moves too far away from the base station 3 it was incommunication with. The handset 11 may now be in range of another basestation 3 of the same system, so that the link could be re-establishedwith this other base station 3 instead of the previous one. Since thetelepoint or exchange system cannot track the movement of the handset,it does not know which base station 3 should be used to re-establish thelink. Therefore, the base station 3 cannot begin transmission.

When a handset 11 begins multiplex 3 transmissions to re-establish alink, these will be received by any base station 3 within range. Thebase station 3 decodes the PID and the LID, and passes these to thecentral controller. This is able to recognise from the PID and LID thatthe handset 11 is trying to re-establish a link it had previouslyestablished with a different base station 3. The central controller canthen instruct the base station 3 now receiving signals from the handset11 to grant the link, and re-connect the handset 11 to the destinationwith which it was previously in communication over a link with the otherbase station 3.

In cases such as small area intercoms and telephone extensions where thehandset 11 is only ever in communication with one particular basestation 3, it is possible for the base station 3 to transmit the firstradio signals to initiate link re-establishment, since there is no needto decide which of several base stations 3 should transmit the signals.However, even in this case there is a benefit in requiring the firstradio signals to be transmitted by the handset 11 and not the basestation 3.

First, the base station 3 will typically be a more powerful transmitterthan the handset 11, and if the base station 3 transmits the first radiosignals these may be received by the handset 11 but the reply from thehandset 11 may not be received by the base station 3. Both the handset 1and the base station 3 will then be active in attempting to re-establishthe link in circumstances which prevent re-establishment from actuallytaking place. If the base station does not transmit until it hasreceived a signal from the handset, it is more likely that the signalstrengths in both directions are adequate for link re-establishment.

Second, if the base station 3 transmits the first signals, it will haveto use CHMF, and all idle handsets 11 within range will have tosynchronise and decode the transmissions before discovering from the PIDwhether the signals are intended for the particular handset 11concerned. If the first signals in link re-establishment are transmittedby the handset 11, using CHMP, and the base station 3 replies usingSYNCF, no other handsets 11 will react to the signals.

Link Quality Checking

The only encoding in the B channel takes place in the encoders 63, 83 ofthe handset 11 and the base 3. As described above, the encoders 63, 83use an adaptive differential pulse code modulation algorithm to performdata compression. They may also reverse the values of selected bits ofthe B channel data according to a predetermined pattern (which will bereversed by the decoders 77, 97), in order to maximise the number of bitreversals in the serial data string. However, the B channel willtypically not include any error detecting or correcting codes. Inparticular, error detecting or correcting codes require the transmissionof code bits, reducing the number of transmitted data bits available tocarry information. The B channel has an average transmitted bit rate of32 kbit per second in each direction, and it is preferable to use all ofthese bits for speech information in order to maximise the quality ofthe transmitted speech.

Therefore, if there are errors in the B channel, the system cannotdetect this fact directly. However, all multiplex data structures carryD channel bits, and errors in the D channel can be detected using theCRC code of the D channel code words. Therefore, the presence of errorsin the B channel during multiplex 1 transmissions can be inferred fromthe detection of errors in the D channel.

Typically, signal errors arise in two ways. First, noise, interferenceand other problems with the radio link and the transmitting andreceiving systems may cause random errors at any average bit error rate.Since these errors are random, each bit position of a multiplexstructure is as likely to suffer such an error as any other. Second,errors may arise from the misinterpretation of the received signal ifthe parts of the radio link lose bit or burst synchronisation. Althoughall bits of a burst are prone to errors due to loss of synchronisation,the first and the last bits of each burst are especially vulnerable. Forthis reason, the D channel bits in multiplex 1.2 and multiplex 1.4 areplaced at either end of the data burst, sandwiching the B channel bits.This ensures that the D channel, in which errors can be detected, ispreferentially vulnerable to errors as compared with the B channel, inwhich errors cannot be detected.

Individual CRC failures in D channel code words are used by the systemcontroller 79, 99 to detect D channel errors so that it can avoid actingon false D channel messages. This may lead to the rejection of D channelpackets and requests for retransmission, using the control octet of thevariable format address code word as described with reference to FIG.29. Additionally, the system controller 79, 99 uses the pattern in whichD channel CRC failures accumulate over time to provide a measure of thequality of the radio link, and if the quality fails to meet a pre-setcriterion, either side can initiate link re-establishment, by sending amessage in the D channel to the other part. In all cases, linkre-establishment is actually carried out by the hand set 11 transmittingin the multiplex 3 data structure.

Serious permanent loss of synchronisation between the parts will lead tocontinuous errors in the D channel and the system controller 79, 99 willrapidly decide that the link quality fails to meet the criterion,whatever criterion is adopted. Accordingly, the link quality criterionshould be chosen in order to provide desired performance when radio linkproblems or slight synchronisation loss cause only some bits to bereceived in error whilst most bits are received correctly.

The effect on the B channel of any average bit error rate can besimulated, and a subjective decision can be taken on what quality ofreceived speech is acceptable. The pattern of CRC failures in the Dchannel for a given bit error rate can also be simulated, and thepatterns for bit error rates leading to acceptable and unacceptablespeech qualities can be compared. On the basis of this comparison, apattern of CRC failures in the D channel can be selected as the linkquality criterion to be used by the system controller 79, 99 in decidingwhether or not to request link re-establishment. Any type of pattern ofCRC errors may be chosen as the link quality criterion, but it has beenfound conveniently simple and effective to specify the criterion as agiven number of successive CRC failures uninterrupted by a D channelcode word in which the CRC check is successful.

Assuming that errors occur in the D channel at random, any given biterror rate will eventually result in an error pattern failing to meetthe quality criterion, and causing link re-establishment. Using wellknown statistical methods, it is possible to calculate the period, forany given bit error rate, during which there is a 50% probability thatan error pattern will occur which fails to meet the quality criterion.

For an ideal criterion, this period should be very short (e.g. afraction of a second) for any bit error rate leading to B channel speechquality which is judged to be unacceptable, so that the link is rapidlyre-established under these circumstances with the minimum disruption tothe telephone conversation being conducted.

On the other hand, for bit error rates permitting excellent speechquality over the B channel, this period should be long compared with thepredicted average length of a call between a handset 11 and a basestation 3, so that unnecessary link re-establishment is unlikely tooccur during calls in which the B channel quality is good. In additionto minimising unnecessary link re-establishments, this reduces thelikelihood that a good quality link will be lost, since there is alwaysa chance that an attempt at link re-establishment will result in loss ofthe link, especially at busy times when most other available channelsare being used for links between other devices.

For intermediate bit error rates, representing B channel speech qualitywhich is less than perfect but which is acceptable at least for shortperiods, the length of time for a 50% probability that linkre-establishment will be attempted will also be intermediate the shortperiod for unacceptable quality and the long period for excellentquality.

FIG. 35 shows a flow diagram of the link quality checking operations ofthe system controller 79, 99. In this case, the quality criterion isthat the number of uninterrupted D channel CRC failures in successionmust not reach N.

When a link is first established, the system controller 79, 99 sets acounter C to 0 in step S1. In step S2 it receives and decodes a Dchannel code word. In step S3 it determines whether the check code andthe parity bit of the D channel code word have the correct values. Ifthe values are correct, the error check succeeds and the systemcontroller returns to S1. Counter C is set to 0, and the qualitymonitoring procedure waits until the next D channel code word isreceived and decoded.

If the CRC code or the parity bit indicate the presence of an error, thecheck in step S3 fails, and the procedure moves to step S4. In thisstep, the value of the counter C is increased by 1. Next, the value ofcounter C is tested in step S5. If the value of C has not yet reached N,the quality monitoring process returns to step S2, and waits for thenext D channel code word to be received and decoded. In this case, theprocedure does not pass through step S1 in returning to step S2, andtherefore the value of C is not reset to 0. If successive D channel codewords contain errors, the link quality monitoring procedure will passround the loop made up by steps S2, S3, S4 and S5, and the value ofcounter C will continue to increase. If at any time a D channel codeword is received without errors, the procedure returns to step S1 andcounter C is reset to 0.

After N successive D channel code words have been received allcontaining errors, the value of counter C will reach N. This will bedetected by the test of the value of C in step S5, and the procedurewill pass to step S6. In this step, it is determined that the link hasfailed to meet the quality criterion, and link re-establishment isinitiated.

If the handset 11 or base station 3 concerned has not received an "IDOK" handshake code during the previous 3 seconds at the time when itreaches step S6 in FIG. 35, it is permitted to attempt to re-establishthe link on a different radio channel. Otherwise, the attempt tore-establish the link must be made on the same radio channel as wasbeing used previously. However, if the errors have arisen through lossof synchronisation between the parts, rather than difficulties withradio transmission and reception, re-establishing the link on the samechannel will normally restore link quality. Additionally, a condition isapplied that link re-establishment on the same channel as was previouslyused is not permitted unless at least 300 ms of transmissions inmultiplex 1.4, or at least 600 ms of transmissions in multiplex 1.2 havetaken place over the link since the link was established or since themost recent link re-establishment.

In an alternative embodiment, the failure of the D channel CRC andparity errors to meet a quality criterion is used as an indication thatthe B channel quality is unacceptably low, but the action taken inresponse to this is not (or not necessarily) to initiate linkre-establishment.

In one alternative, the device (handset 11 or base station 3) detectingthe errors reacts by muting the B channel, so that the user hearsnothing instead of hearing the poor quality B channel, but linkre-establishment is not attempted until there is a loss of handshake(i.e. "ID OK" has not been received) for three seconds, as describedabove.

In another alternative, the device reacts by initiating a change frommultiplex 1 to multiplex 2. Because of the increased amount of D channeland the presence of the S channel, it is easier to maintain contact overa low quality link in multiplex 2 than in multiplex 1.

In both cases, a temporary reduction in link quality will result in acorresponding temporary cessation of B channel communication, but thelink is maintained and B channel communication may be restored when thelink quality recovers. In both cases, it is possible as a further optionto initiate link re-establishment if it is not possible to restore Bchannel communication within a time-out period.

D Channel Fill In

In the fixed format address code word shown in FIG. 28, the PID and LIDcodes may have any values, in accordance with the identity codes whichhave been given to particular devices or types of service. Similarly,the message contents octets of the variable format address code word ofFIG. 29 and the data code word of FIG. 30 may adopt any value, dependingof the D channel message being sent. Therefore, there is the possibilitythat the contents of a D channel word may, by chance, resemble the SYNCD pattern. If this happens, the system controller 79, 99 may believethat it has received SYNC D when it has really received part of a Dchannel code word. Therefore the D channel decoding by the systemcontroller will not be properly synchronised with the received D channeldata, and the D channel data will be misinterpreted.

In most cases, this error is self-limiting. Every address code word inthe D channel must be immediately preceded by SYNC D. If the systemcontroller 79, 99 is out of synchronisation with the D channel, it willprobably not find the SYNC D pattern when it expects this pattern toappear again. When this happens, the system controller 79, 99 willabandon its incorrect synchronisation with the D channel, and willsearch the D channel for the SYNC D pattern, which will enable it tore-establish correct synchronisation.

However, a problem can arise if successive address code words in the Dchannel contain patterns which resemble SYNC D, at the same relativepositions in the code words. In this case, the system controller 79,99can become locked into the incorrect D channel synchronisation. In orderto avoid this, successive address code words must be spaced by 48 bitsof IDLE D, unless it is guaranteed that the address code words do notcontain a pattern resembling SYNC D or the two successive address codewords are sufficiently different that they cannot carry patternsresembling SYNC D at the same relative position.

D channel code words must be transmitted with sufficient frequency topermit satisfactory channel quality monitoring using the CRC code andthe parity bit in each code word. If there are a large number of Dchannel messages to be sent, this requirement is met by the almostcontinuous stream of code words required to carry the messages,interleaved with fixed format address code words to carry handshakesignals as required. However, if the same D channel message is to besent repeatedly, so that the same variable address code word is sentrepeatedly, or if no D channel messages are to be sent so that only thefixed format address code word (which will be the same every time) is tobe sent, the address code words must be spaced by 48 bits of IDLE D toavoid the possibility of locking onto false D channel synchronisation,as discussed above. IDLE D is not a D channel code word, but simply apattern of alternating "1"and "0" bit values and it does not include aCRC code. In view of the slow rate at which D channel data istransmitted in multiplex 1, this requirement to send 48 bits of IDLE Dmay mean that the rate at which D channel code words is sent isinsufficient for satisfactory channel quality monitoring.

To solve this problem, a special "FILL-IN" D channel code word isdefined. This code word is a "supervisory type" variable length formataddress code word, not followed by any data words and carrying a specialmessage in all "contents" octets (i.e. octets 3, 4, 5 and 6) which isdefined to have no meaning. The FILL-IN code word is designed so that nopart of it, including the check code in octets 7 and 8, resembles theSYNC D sequence. Therefore, the FILL-IN word can be sent continuously,to maintain the D channel error check rate, when there are no D channelmessages to be transmitted. Because it is known not to contain a falserepresentation of SYNC D, there is no need to precede repeats of thisword by 48 bits of IDLE D. Additionally, if it is desired for any reasonto transmit another address code word repeatedly, the FILL-IN word maybe interleaved with the other address code word, in place of the 48 bitsof IDLE D, to provide a guarantee that the receiving system controllercannot lock into a false D channel synchronisation, while at the sametime the FILL-IN word maintains the rate of D channel code words for usein channel quality monitoring.

FIG. 36 shows the bit pattern of octets 1 to 6 for a D channel wordwhich is suitable for use in a system with a SYNC D pattern of"0010001111101011". The "X" given at bits 1, 2 and 4 of octet 2 indicatethat these bits may be either "1" or "0". The pattern "11110000" inoctets 3 to 6 is effectively a "supervisory type" message of no meaning.This same bit pattern is used in the last code word of an "informationtype" packet to fill up octets not used by the message beingtransmitted.

S Channel Word Structure

The S channel synchronisation words, SYNCP, SYNCF, CHMP and CHMF areused during multiplex 3 and multiplex 2 transmissions to enable thereceiving device to obtain burst synchronisation with the transmittingdevice. Until the relevant synchronisation word has been detected, theprogrammable demultiplexer 75,95 cannot achieve burst synchronisationwith the incoming data, and it is not possible to interpret the Dchannel.

Since the S channel synchronisation word must be detected before burstsynchronisation can be achieved, it must be possible to detect the wordasynchronously. For this reason, once bit synchronisation has beenachieved, each bit of the incoming data is passed to the S channelcontroller 81,101, and in each bit period the S channel controllercompares the pattern of the 24 most recently received bits (assumingthat the S channel synchronisation words are 24 bits long) with thestored target word patterns. In order to allow the S channelsynchronisation words to be detected in the presence of a small amountof noise, the S channel controller provides a "word found" output if theinput bits match the target pattern for at least 22 of the 24 inputbits.

In order to avoid the possibility that the receiving device willincorrectly identify the presence of an S channel synchronisation word,and therefore obtain incorrect burst synchronisation, it should ideallynot be possible to obtain in multiplex 2 or multiplex 3 a pattern ofdata which, if correctly received, provides a match to 22 of the 24 bitsof any synchronisation word, except when the synchronisation word itselfappears in the data and is compared with the stored synchronisation wordin precisely the correct alignment. Thus, if the received S channelsynchronisation word is compared with the stored version of itself, butmisaligned by one or more bit periods, or if any other part of themultiplex 2 or multiplex 3 transmissions are compared with the stored Schannel synchronisation word, there should be no recognition that thesynchronisation word is present, or else incorrect burst synchronisationwill result.

These requirements may be considered in general terms for asynchronisation word of length L, used in a system in which recognitionof the presence of the synchronisation word is deemed to have occurredif a comparison between the bit pattern of the word and the bit patternof incoming data gives no more than K errors for any string of L databits in the incoming signal. In this generalised case, the followingconditions are each individually helpful, and are preferably bothpresent.

A) Each data burst has fixed and variable portions, and each possiblestring of L consecutive bits contains fewer than L-K variable bits. Itis assumed that the variable portions can assume any values, and sothere is the possibility that by chance L successive bits of variabledata will provide precisely the same pattern as the S channelsynchronisation word. By splitting the variable data so that Lconsecutive bits contain less than L-K variable bits, the possibility isavoided that L-K bits of such a pattern of variable data may occur as anunbroken bit stream in the data burst, leading to false recognition ofthe S channel synchronisation word.

In the multiplex 2 structure, the D channel portions are variableportions and the S channel portion (preamble plus S channelsynchronisation word) is a fixed portion. Although the multiplex 2 burstcarries 32 bits of D channel data, this is split into two 16-bitportions separated by 34 bits of the S channel, so that the D channelalone can never mimic 22 or more bits of the 24-bit S channelsynchronisation word. The amount by which the maximum number of variablebits in any string of L consecutive bits is less than L-K can beregarded as a protection factor. Thus, if the number of variable bits isone bit less than L-K bits, it provides a protection factor of one bit.If it is two bits less, it provides a protection factor of two bits. Inview of the possibility that errors in the received data may cause thefixed data burst portion next to the variable data burst portion tomimic the extra bits of the S channel synchronisation word which cannotbe contained in the variable bits because of their fewer number, ahigher protection factor gives better protection against the possibilitythat chance resemblance of variable data to the S channelsynchronisation word may lead to incorrect burst synchronisation. In thecase of multiplex 2, L-K is 22 whereas any string of 24 consecutive bitscan only include a maximum of 16 variable bits, providing a protectionfactor of 6 bits.

B) Any string of L bits made up wholly of a fixed data portion of aburst must give more than K errors when compared with any of thesynchronisation word patterns. This applies to all strings of L bits offixed data, including strings containing part of a synchronisation worditself, except for the string made up precisely of the correctsynchronisation word in its correct position. Additionally, any stringof L bits made partially of fixed data and partially of variable data,must also give more than K errors when compared with any of thesynchronisation word patterns, while assuming that the variable databits give no errors. Again, it is possible to define a protectionfactor. In this case, the protection factor is the number of errors inexcess of K provided on this basis by the string of L bits of fixed dataor part fixed and part variable data which gives the least number oferrors in comparison with any of the synchronisation word patterns.

Theoretically, it is possible to discover the patterns of L bits whichmeet condition B) for any given burst structure or, if there are no suchpatterns, to discover the patterns which come closest to meetingcondition B), by comparing all possible patterns of L bits with theburst structure at all possible bit offset positions. In practice, forany reasonably large value of L, there are so many possible L-bitpatterns that such a comparison cannot be carried out in a reasonableamount of time. However, since the synchronisation word itself forms allor part of the fixed portion of the data burst, the degree ofself-correlation between each synchronisation word pattern and itselfoffset by one or more bits, and the cross-correlation, with or withoutoffset, between different synchronisation word patterns, are bothrelevant to condition B) above.

In the worst case, it can be assumed that the S channel synchronisationword is embedded in the multiplex structure in variable data. Underthese circumstances, if M is the number of matches of thesynchronisation word with itself offset by S bits, then for all valuesof S, M+S must be less than L-K, to meet condition B) above. The amountof the offset S is added to the number of matches M, to take account ofthe possibility that all bits of the variable data may by chance matchprecisely with the bits of the synchronisation word they are comparedwith. As S approaches L, so that the synchronisation word is offset tosuch a high degree that it only overlaps itself by a very few bits, thiscondition becomes harder to meet. However, condition A) above prevents Sfrom reaching L-K, as it prohibits the presence of this much variabledata in L successive bits.

For any reasonably high value of L (i.e. for any reasonably longsynchronisation word) it can be extremely arduous to find all possiblepatterns of length L which meet this condition, or to find the patternsof length L for which the highest value of M+S remains below L-K by thegreatest amount. Nor is it necessarily appropriate to do so, as it ispossible to use burst structures such as multiplex 2 and multiplex 3 inwhich the S channel synchronisation word is not embedded in variabledata. In multiplex 3 the S channel synchronisation word is provided with12 bits of preamble on either side and in multiplex 2 it has 10 bits ofpreamble in front of it and the variable data behind it is restricted to16 bits. As a practical matter, it is reasonable to assume that bitpatterns having low self-correlation and cross-correlation side lobeswill tend to be suitable for condition B) above, as they will tend tohave low values of M.

The value of a self-correlation side lobe in the comparison of a patternwith itself at an offset of S bits is defined, for the purposes of thispatent application, as being the number of matches in the comparison ofthe pattern with itself at this offset, minus the number of mismatches.If the value of the self-correlation side lobe is calculated for allvalues of S (except S=0: correct alignment), the maximum of all thevalues of the self-correlation side lobes found for all the values of Scan be taken as a measure of the degree of self-correlation of the bitpattern. The lower this value, the more promising the pattern is as acandidate for an S channel synchronisation word.

The cross-correlation side lobes are defined in the same way, for thecomparison of one synchronisation word pattern with another, except thatin this case the value at S≦0 must also be taken into consideration.

In the design of the illustrated embodiment, condition A) was met by thedesign of the multiplex 2 and multiplex 3 burst structures. In thiscase, L, the length of the S channel synchronisation word, is 24 and K,the number of permitted errors in recognising the S channelsynchronisation word, is 2. In any continuous string of 24 bitstransmitted in multiplex 2 or multiplex 3, the maximum number of Dchannel bits which can be present is 16, giving a protection factor of 6bits for condition A).

It was decided to seek to meet condition B) by appropriate choice of bitpatterns for the S channel synchronisation words. The preamble bits inthe S channel in multiplex 2 and multiplex 3 are present to enable thereceiving device to obtain bit synchronisation, and it might interferewith this objective if this bit pattern was altered to improveperformance on condition B) with an arbitrarily chosen S channelsynchronisation word pattern. The preamble bits in the D channel inmultiplex 3 are also useful for enabling bit synchronisation, andadditionally the pattern is the same as the pattern for IDLE D, so thatit is unlikely to result in misinterpretation of D channel data.Therefore, it was also considered to be undesirable to attempt tomanipulate these bit patterns to improve performance on condition B).

In order to simplify calculations, it was decided first to identify goodcandidate patterns for the synchronisation words, by selecting thosewith good self-correlation properties, i.e. those with low values forthe highest value self-correlation side lobe. All possible 24 bit binarypatterns were examined to identify those with good self-correlationproperties by this definition.

Because of the large number of calculations involved, the candidatepatterns were not tested to ensure that condition B) was met for everypossible string of 24 bits in multiplex 2 and multiplex 3. Instead, amultiplex 3 test and an S channel test were used.

In the multiplex 3 test, a 24 bit candidate pattern was compared withthe 8 preamble, 10 data, arrangement of bits used in the D channel inmultiplex 3, at each of the 18 possible different bit offsets. After 18bit positions of offset, the arrangement of bits repeats, so thatcomparison at further offsets is unnecessary. The result of the test wasthe maximum number of matches obtained with any comparison bit pattern.To satisfy condition B) on this test, the maximum number of matches hadto be less than L-K, i.e. less than 22. In accordance with condition B),it was assumed that every bit in the D channel provided a perfect matchto the corresponding bits in the 24 bit candidate pattern.

The candidate pattern is aligned with the maximum number of variabledata bits when it is aligned with one 8 bit preamble portion and part ofeach of the 10 bit data portions on either side of the preamble portion.In this case, the 24 bit pattern is aligned with 16 variable bits. Thisis the same as the maximum number of variable data bits a pattern can bealigned with in multiplex 2, and it is believed that the separation ofthe variable bits in multiplex 3 into two portions of up to 10 bits,rather than one portion of 16 bits as in multiplex 2, provides a morestringent test for the 24 bit candidate pattern.

Additionally, in multiplex 2 it takes two bursts to transmit a D channelcode word, and as has been described above with reference to the FILL-INword, there are rules which prevent the continuous unbroken repetitionof the same D channel code word. Therefore the variable data portions ofmultiplex 2 bursts will be different from burst to burst, and will notrepeat for several bursts. Thus any variable data pattern in a multiplex2 burst leading to incorrect recognition of the S channelsynchronisation word will not be repeated, and a recovery from incorrectrecognition to correct recognition should occur rapidly. In multiplex 3the variable (D channel) portions tend to be the same from burst toburst, making recovery from incorrect recognition of the S channelsynchronisation word more difficult. Therefore the avoidance ofincorrect recognition is more important in multiplex 3 than in multiplex2.

For these reasons, it was not considered necessary to carry out acorresponding test using the multiplex 2 pattern of variable data bits.

The best self-correlation performance found amongst all possible 24 bitbinary patterns provided a value of +1 as the highest self-correlationside lobe value. Some of these values also met condition B) in themultiplex 3 test. However, all of these provided at least one positionin the multiplex 3 test where 21 matches were possible. That is to say,these patterns provided a protection factor for condition B) of only onebit in the multiplex 3 test.

It was considered that this was not satisfactory, as this meant that afalse recognition of an S channel synchronisation word would be possiblewith some particular pattern of D channel data in multiplex 3 if asingle error arose in the reception of the data. Since the D channeldata transmitted in multiplex 3 is the PID and LID codes, this meantthat a handset 11 with a particularly unfortunate PID code might sufferan excessive rate of failure when attempting to initiate a link, becausethe base station would be prone to mis-identify part of the D channel asthe S channel synchronisation word in the presence of a small degree ofnoise, and therefore fail to decode the multiplex 3 transmissionscorrectly. Therefore all candidate patterns having a maximumself-correlation side lobe value of +1 were rejected, and candidatepatterns having a maximum self-correlation side lobe value of +2 wereconsidered. Several such codes were found which provided a protectionfactor of 2 bits for condition B) in the multiplex 3 test.

By accepting candidate bit patterns with self-correlation side lobes ofup to +2, the chance is increased that incorrect burst timing will beobtained in a noisy environment, by misinterpreting S channel dataoffset from the correct S channel synchronisation word timing. However,it was considered that this situation was preferable to the possibilitythat some particular devices would suffer the problems which might flowfrom a protection factor of only 1 bit in the multiplex 3 test.

Pairs of bit patterns were identified which gave a bit protection factorof 2 for condition B) in the multiplex 3 test, such that each member ofa pair was the bit inverse of the other. The use of bit inverse pairsmeans that it is unnecessary to consider the effect of the polarity ofthe D channel and S channel preamble portions. Seven such codes,fourteen such patterns, were found. Each of these patterns was comparedwith each of the others, including its bit inverse, to determinecross-correlation values. The 2 bit inverse pairs of patterns having thelowest maximum value of cross-correlation lobes for the 6cross-correlations between them were selected.

The S channel test was carried out on the selected patterns. In thistest, each of the four codes was compared at all offsets with each offour comparison patterns. The comparison patterns were 36 bits long, andconsisted of 12 preamble bits and then a respective one of the fourcandidate patterns. The comparison pattern is the same as the structureof one repeat in the S channel submultiplex of multiplex 3. It alsoincludes within it the pattern of the S channel in multiplex 2.Therefore this test provides an indication of the probability that Schannel data in multiplex 2 or multiplex 3 will be incorrectlyidentified as the wrong S channel synchronisation word, or as the rightword with the wrong timing.

In the S channel test, when each candidate pattern is compared with thetest pattern containing itself, there will be a complete 24 bit matchwhen the candidate pattern is aligned with itself in the test pattern.This data is irrelevant, since it represents a correct decoding of the Schannel, rather than an incorrect decoding, and therefore it wasdiscarded. After this irrelevant data had been discarded, the resultsfor each bit inverse pair were studied to determine the alignment withany of the test patterns giving the greatest number of matches. For onebit inverse pair, this greatest number was 15 matches and for the otherthe greatest number was 14 matches.

Thus, the S channel test gave protection factors of 7 bits and 8 bitsrespectively for condition B).

The bit inverse pair giving a maximum of 14 matches, and a protectionfactor of 8 bits, were selected as the channel marker codes CHMF andCHMP, and the other pair were selected as the ordinary S channelsynchronisation words SYNCF and SYNCP.

It should be noted that the protection factors for condition B) given bythe multiplex 3 test and the S channel test only apply to the particulargroup of alignments between the synchronisation word and the burststructure for which the tests are carried out, and do not necessarilyguarantee that these protection factors are provided for all possiblealignments between the synchronisation word and the burst structure.

However, the multiplex 3 test replicates the arrangement of bitsthroughout the first four submultiplexes of multiplex 3, and the Schannel test replicates the arrangement of bits in the fifthsubmultiplex of multiplex 3. At the transition between the fourth andfifth submultiplexes, ten bits of variable D channel data are followedby fourteen preamble bits (two from the D channel in the fourthsubmultiplex and twelve from the S channel in the fifth submultiplex)before the twenty four W bits making up the S channel synchronisationword. It is valid to treat preamble and W bits (which are fixed) asparticular values for variable bits in order to fit this arrangement tothe arrangement used in the multiplex 3 test. Therefore, the results ofthe multiplex 3 test and the S channel test between them provide aguarantee that a protection factor of at least two bits is available forcondition B) at all possible alignments of the synchronisation word andthe multiplex 3 burst structure.

The values chosen for the S channel synchronisation words, expressed inhexadecimal and binary notation, are as follows:

    ______________________________________                                        CHMF:         BE4E50 hexadecimal;                                                           101111100100111001010000 binary                                 CHMP:         41B1AF hexadecimal;                                                           010000011011000110101111 binary                                 SYNCF:        EB1B05 hexadecimal;                                                           111010110001101100000101 binary                                 SYNCP:        14E4FA hexadecimal;                                                           000101001110010011111010 binary                                 ______________________________________                                    

As will be appreciated by those skilled in the art similar performancecould be obtained with a set of 4 bit patterns for the S channelsynchronisation words which were the same patterns as given above inreverse bit order. These patterns would be 0A727D, F58D82, A0D8D7 and5F2728 in hexadecimal notation.

As a further test for the bit patterns selected as the synchronisationwords, their performance when embedded in variable data was examined. Itwas found that for each of the selected bit patterns, there was at leastone number S of bit positions by which the pattern is offset from itscorrect position at which M+S is equal to or greater than L-K. That isto say, if it is assumed that all of the bits of variable data provide aperfect match, there is at least one offset value S at which the totalnumber of matches to the 24 bit synchronisation pattern is at least 22.As mentioned above, this is in fact inevitable once S reaches 22.

The bit patterns selected for the synchronisation words are neverthelesssuitable in practice. In the multiplex 3 data structure, thesynchronisation word is never embedded in variable data, but is alwayspreceded by 12 bits of preamble, and is either followed by 12 bits ofpreamble or, in the case of the last repeat in the fifth submultiplex,is followed by the end of transmission. As noted above, there is aguarantee that false recognition cannot occur in multiplex 3 unless atleast two bits are wrongly received (e.g. due to noise). In multiplex 2,the S channel synchronisation word is always preceded by 10 bits ofpreamble, and is followed by only 16 bits of variable D channel databefore the end of transmission. Therefore, even allowing for the twopermitted errors in recognition, offset values of greater than 18 forwhich the synchronisation word is embedded entirely in variable data,which lead to the greatest probability of false recognition, cannotoccur.

Furthermore since the 10 preamble bits in multiplex 2 come before thesynchronisation word, and the 16 D channel bits adjacent to thesynchronisation word come after it, the adjacent 16 D channel bits canonly lead to false recognition of the S channel synchronisation word ifthe S channel controller 81, 101 has failed to detect thesynchronisation word when it actually occurred. If the S channelcontroller 81, 101 recognises the presence of the synchronisations wordwhen it does occur, the frame timing controller 153 will use the timingof this recognition to set the frame clock 155, and will ignore afurther recognition signal from the synchronisation word recognisers137, 139 should such a further, erroneous, output be provided a few bitperiods later.

Finally, occasional incorrect recognition in multiplex 2 does notmatter, provided that its frequency is low, since recovery fromincorrect recognition tends to happen anyway in multiplex 2 as explainedabove.

In order to determine that the likelihood of such an erroneousrecognition was acceptably low, it was assumed that the synchronisationword was embedded in randomly variable data. In this case, theprobability that false recognition will occur at an offset of anyparticular number S of bit periods is the probability that S bits ofrandom data will provide at least (N-K-M) matches, where M is a numberof matches which the synchronisation word has with itself at an offsetof S bits, N is the total length of the synchronisation word (i.e. 24),and K is the number of errors permitted in successful recognition (i.e.2).

For any given value of S, this probability is: ##EQU1## is a binominalcoefficient.

The sum of these probability values for all values of S, i.e. from S=1to S=23, provides a figure for the probability or frequency of a falsedetection output if the synchronisation word is embedded in random data.Cross correlation values, i.e. figures for the false recognition of onesynchronisation word when another is embedded in random data, may beprovided using the same formula, but the probability for zero offset,i.e. S=0, should also be included.

Tables 1, 2 and 3 below give the peak side lobe value, peak number ofmatches and false detection value for each of the synchronisation wordswhen compared with itself and when compared with each of the othersynchronization words and also the "0101 . . . " preamble pattern.

                  TABLE 1                                                         ______________________________________                                        Side Lobes                                                                    CHMF        SYNCF     CHMP    SYNCP  0101 . . .                               ______________________________________                                        CHMF    2       10        6     7      4                                      SYNCF           2         7     6      5                                      CHMP                      2     10     4                                      SYNCP                           2      5                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Matches                                                                       CHMF        SYNCF     CHMP    SYNCP  0101 . . .                               ______________________________________                                        CHMF    12      16        11    13     12                                     SYNCF           11        13    12     12                                     CHMP                      12    16     12                                     SYNCP                           11     12                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        False Detection Values                                                        CHMF        SYNCF     CHMP    SYNCP  0101 . . .                               ______________________________________                                        CHMF    2.7E-5  2.89E-4   1.50E-3                                                                             4.80E-4                                                                              2.77E-4                                SYNCF           6.9E-5    4.80E-4                                                                             1.42E-3                                                                              5.95E-4                                CHMP                      2.7E-5                                                                              2.89E-4                                                                              1.40E-4                                SYNCP                           6.9E-5 3.55E-4                                ______________________________________                                    

When comparing tables 1 and 2, it should be noted that the peak sidelobe value and the peak number of matches for any comparison do notnecessarily occur at the same amount S of offset. In table 3 "E" standsfor "exponential", and means that the first number should be multipliedby 10 to the power of the second number. Thus 1.42E-3 means 0.00142.

For comparison, it may be noted that the 24 bit pattern111100001111000011110000 has a peak self correlation side lobe value of16, its greatest number of matches at any offset is 18, and its falsedetection value is 1.47E-1 or 0.147 (i.e. assuming that it was embeddedin random data, it would cause an incorrect recognition output foritself having a wrong timing on 14.7 percent of occasions in which thepattern appeared).

Modifications and Alternatives

FIG. 37 is a schematic diagram, similar to FIG. 1, showing atelecommunications network 1 having network links 9 to modified basestations. Base station 189 has a single network link 9 to thecommunications network 1, and resembles the base stations 3 of FIG. 1 inthis respect. However, it has a distributed antenna 191, e.g. a "leakyfeeder", in place of the conventional aerial 43 of the base station 3.This may permit improved geographical coverage by the base station 189,with relatively low power radio transmissions.

Base station unit 193 has a plurality of network links 9 to thetelecommunications network 1, and can therefore connect a plurality ofhandsets 11 to the telecommunications network 1 over respective networklinks 9 and respective radio links 13, on different radio channels. Thebase station unit 193 may be constructed as shown schematically in FIG.38. A plurality of base station control circuits 55, each similar to thecircuit described with respect to FIG. 16, are connected to respectivenetwork links 9 by respective telephone connections 45. Thetransmit/receive switches 91 are connected to a radio signal combiner195, instead of to respective aerials 43. Through the action of thecombiner 195, each individual control circuit 55 can transmit andreceive using a common aerial 197. In order to prevent the transmissionsby one control circuit 55 disrupting receiving operations of anothercontrol circuit 55, the burst timing for all of the base station controlcircuits 55 of the base station unit 193 is controlled centrally, sothat they transmit and receive synchronously.

The arrangement of FIG. 39 provides a similar operation to thearrangement of FIG. 38, but in this case each base station controlcircuit 55 has a separate respective aerial 43, in place of the combiner195 and common aerial 197. Thus, the arrangement of FIG. 39 closelyresembles a collection of base stations 3, each having a single networklink 9, in close proximity. However, in view of the close proximity ofthe units, and in particular the close proximity of their aerials 43, itwill normally be necessary to ensure burst synchronisation between therespective base station control circuits 55, so that transmissions fromone circuit do not swamp an attempt by another circuit to receive asignal from a handset 11.

FIG. 40 shows a further modification, which permits a base station unitto connect a plurality of handsets 11 for intercom communication, or toconnect a plurality of handsets 11 to a single network link 9 to providea conference call. A plurality of base station control circuits 199 areprovided. In FIG. 40, they are shown as each having a respective basestation aerial 43, as in FIG. 39, but a combiner 195 and common aerial197 could be provided instead. Each base station control circuit 199 isconnected to a switching circuit 201, which is in turn connected to oneor more network links 9 by respective telephone connections 45. Thenumber of network links 9 to which the switching circuit 201 isconnected may be fewer than the number of base station control circuits199. The switching circuit 201 operates under the control of signalsreceived from the base station control circuits 199, to connectrespective base station control circuits 199 together and/or connectthem to a telephone connection 45, so as to provide intercom/conferencefacilities in addition to normal telephone call facilities.

Each base station control circuit 199 may be the same as the basestation control circuit 55 described with respect to FIG. 16. In thiscase, the switching circuit 201 would receive signals from, and sendsignals to, the respective line interfaces 103 of the control circuits.However, it is preferable to provide the base station control circuits199 as modifications of the structure shown in FIG. 16, in which theencoder 83, the decoder 97 and the line interface 103 are not present.Instead, the switching circuit 201 contains an encoder, a decoder and aline interface for each of its telephone connections 45.

In this case, the switching circuit 201 receives the B channel datadirect from the programmable demultiplexer 95, and provides this data tothe decoder 97 of the respective telephone connection 45 if the signalsare to be sent over a network link 9, and provides the B channel signalsfrom the programmable demultiplexer 95 of one base station controlcircuit 199 to the programmable multiplexer 85 of another base stationcontrol circuit 199 if they are to be transmitted to a further handset11. The signals from the system controller 99, normally sent to the lineinterface 103, may be used to control a switching control unit whichcontrols the operations of the switching circuit 201, or may be passedon to a line interface 103 associated with a telephone connection 45 orto the system controller 99 of a further base station control circuit199, as appropriate. As in the arrangement of FIGS. 38 and 39, the bursttiming of the operations of the base station control circuits 199 shouldbe synchronised by providing a common burst timing signal.

In any of the base station arrangements, the aerials 43,197 may bereplaced by a distributed antenna 191 as shown in FIG. 37.

The preferred embodiments of the present invention have been describedlargely on the assumption that a radio link is set up between a basestation and a handset in order to permit speech conversation over the Bchannel. However, as mentioned with reference to FIG. 15, the handset 11may be incorporated in a personal computer or portable computerterminal, to enable the radio link to carry computer data signals. Inthis case, the computer data signals may be carried by the B channel inmultiplex 1, or alternatively the radio link may never move to multiplex1 and the computer data may be carried as special messages in the Dchannel using multiplex 2. Data communications using the B channel andmultiplex 1 will be considerably faster, as each multiplex 1transmission burst carries 64 bits of B channel, all of which areavailable to carry the data. In multiplex 2, only 32 bits of D channelare carried per burst, and additionally the code word structure used forcarrying D channel messages means that only about half of the D channelbits are available for carrying the computer data. However, the use ofthe D channel to carry computer data may be advantageous in somecircumstances, since the D channel transmissions are encoded for errordetection. If a radio link is used to communicate computer data via theD channel, the two parts may communicate only in multiplex 2 once thelink has been set up, and link transmissions may never switch tomultiplex 1.

In another modification, a handset 11 may be provided without a keypad31, or with only a few keys, so that a telephone number cannot bedialled from the handset 11. Such a handset may be used only to receivecalls, or may be permitted to make calls only to one or a fewpreselected numbers. The numbers may be stored in the handset andtransmitted automatically to the base station. Alternatively, especiallyif there is only one number, it may be stored by the base station andselected in response to the PID of the handset.

The embodiments described above are provided by way of example, andvarious modifications and alternatives will be apparent to those skilledin the art.

We claim:
 1. A telecommunication system comprising:a device of a firsttype and a plurality of devices of a second type, said device of thefirst type comprising communication means for conducting time-divisiontwo-way communication with any one of said devices of the second typeover a radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the devices is completed before transmission of the next of saidalternating bursts by the other of the devices is begun, and each ofsaid devices of the second type comprising communication means forconducting said time-division two-way communication with the device ofthe first type; the device of the first type comprising means forproviding said bursts such that at least some of the bursts contain afirst synchronisation pattern or one of a group of first synchronizationpatterns, and detecting means for asynchronously detecting a secondsynchronisation pattern or one of a group of second synchronisationpatterns in a received burst to enable it to determine timinginformation about said received burst, each device of the second typecomprising means for producing said bursts such that at least some ofthe bursts contain said second synchronisation pattern or one of saidgroup of second synchronisation patterns, and detecting means forasynchronously detecting said first synchronisation pattern for one ofsaid group of first synchronisation patterns in a received burst toenable it to determine timing information about said received burst,said first synchronization pattern or patterns being different from saidsecond synchronization pattern or patterns, said detecting means in theplurality of devices of the second type not responding to reception ofthe second synchronisation pattern or patterns, whereby each of saidplurality of devices of the second type does not respond to reception oftransmissions or any other device of the second type.
 2. A method oftelecommunication comprising the steps of:establishing time-divisiontwo-way communication over a radio channel between a device of a firsttype, acting at a given time as one of a receiver and a transmitter, andany one of a plurality of devices of a second type, acting at said giventime as the other of said receiver and transmitter, by exchanging radiosignals in alternating bursts carrying digital data, such that duringthe time-division two-way communication, transmission of one of saidalternating bursts from one of the devices is completed beforetransmission of the next of said alternating bursts by the other of thedevice is begun; producing said bursts such that at least some of thebursts contain a synchronisation pattern; asynchronously detecting saidsynchronisation pattern by the receiver to enable it to determine timinginformation about the received bursts, the device of the first typetransmitting a first synchronisation pattern or one of a group of firstsynchronisation patterns and the devices of the second type transmittinga second synchronisation pattern or one of a group of secondsynchronisation patterns, the first synchronisation pattern or patternsbeing different from the second synchronisation pattern or patterns; andpreventing the plurality of devices of the second type from respondingto reception of the second synchronisation pattern or patterns, wherebyeach of said plurality of devices of the second type does not respond toreception of transmissions by any other device of the second type.
 3. Asystem according to claim 1 wherein said time-division two-waycommunication may be performed by any one of a plurality of devices ofthe first type, and detecting means in the plurality of devices of thefirst type do not respond to reception of the first synchronisationpattern or patterns, whereby each of the plurality of devices of thefirst type does not respond to transmissions by any other device of thefirst type.
 4. A system according to claim 1 or claim 3 wherein theburst producing means of each said device of one type transmits apredetermined synchronisation pattern while attempting to initiatecommunication by said radio signals with a device of the other type, andsubsequently transmits a different predetermined synchronisation patternafter said communication has been initiated.
 5. A telecommunicationsystem comprising:first and second devices having communication meansfor providing time-division two-way communication between said devicesover a radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the first and second devices is completed before transmission of thenext burst by the other of the first and second devices is begun; meansfor producing bursts such that at least some of the bursts transmittedby a said device are bursts of a particular format and contain asynchronisation pattern of L bits of data and also bits of variabledata; means for asynchronously detecting said synchronisation pattern bythe receiver to enable it to determined the timing information about thetransmitted bursts of said particular format; means in the receivingdevice which determines the synchronisation pattern to be present in thereceived data when a comparison operation between the received data anda stored copy of the synchronisation pattern results in no more than Kbits of the received data failing the comparison, where K is zero or apositive integer; and the arrangement of bits in each said burst of saidparticular format being such that in any consecutive string of L bits ofdata in the burst, there are less than L-K bits of variable data.
 6. Amethod of telecommunication in which first and second devicescommunicate with each other, comprising the steps of:performingtime-division two-way communication between said devices over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communication,transmission of one of said alternating bursts from one of the first andsecond devices is completed before transmission of the next of saidalternating bursts by the other of the first and second devices isbegun; producing said bursts such that at least bursts of a particularformat transmitted by a said device contain a synchronisation pattern ofL bits of data and also bits of variable data; asynchronously detectingsaid synchronisation pattern by the receiver to enable it to determinetiming information about the transmitted bursts of said particularformat; determining in the receiver that the synchronisation pattern ispresent in the received data when a comparison operation between thereceived data and a stored copy of the synchronisation pattern resultsin no more than K bits of the received data failing the comparison,where K is zero or a positive integer; and arranging the bits in eachsaid burst of said particular format such that in any consecutive stringof L bits of data in the burst of said particular format, there are lessthan L-K bits of variable data.
 7. A system according to claim 5 whereinthe arrangement of bits in each said burst of said particular format issuch that in any consecutive string of L bits of data in the burst ofsaid particular format, there are no more than L-K-6 bits of variabledata.
 8. A telecommunication system comprising:first and second deviceshaving communication means for providing time-division two-waycommunication between said devices over a radio channel by exchangingradio signals in alternating bursts carrying digital data, such thatduring the time-division two-way communication, transmission of one ofsaid alternating bursts from one of the first and second devices iscompleted before transmission of the next of said alternating bursts bythe other of the first and second devices is begun; means for producingbursts including alternating bursts, such that at least some of thebursts produced by said means for producing bursts are bursts of aparticular format of digital data and comprise a first portion having arepeating pattern of fixed value and variable bits and a second portioncomprising an L-bit synchronisation pattern; means for asynchronouslydetecting said synchronisation pattern by the receiving device to enableit to determine timing information about the bursts of said particularformat; and means in the receiving device which determines thesynchronisation pattern to be present in the received data when acomparison operation between the received data and a stored copy of thesynchronisation pattern results in no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer; theL-bit synchronisation pattern and the repeating pattern of fixed valueand variable bits being such that a string of L successive bits of therepeating pattern, starting at any position in a repeat of the pattern,matches less than L-K bits of the synchronisation pattern even if it isassumed that every variable bit in the string provides a match.
 9. Amethod of telecommunication in which first and second devicescommunicate with each other, comprising the steps of:performingtime-division two-way communication between said devices over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data, such that during the time-division two-way communication,transmission of one of said alternating bursts from one of the first andsecond devices is completed before transmission of the next of saidalternating bursts by the other of the first and second devices isbegun; producing bursts such that at least bursts of a particular formatof digital data comprise a first portion having a repeating pattern offixed value and variable bits and a second portion comprising an L-bitsynchronisation pattern; asynchronously detecting said synchronisationpattern by the receiver to enable it to determine timing informationabout the bursts of said particular format; determining thesynchronisation pattern to be present in the received data when acomparison operation between the received data and a stored copy of thesynchronisation pattern results in no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer; andselecting the L-bit synchronisation pattern and the repeating pattern offixed value and variable bits such that a string of L successive bits ofthe repeating pattern, starting at any position in a repeat of thepattern, matches less than L-K bits of the synchronisation pattern evenif it is assumed that every variable bit in the string provides a match.10. A system according to claim 8, wherein any said string of Lsuccessive bits of the said repeating pattern matches no more than L-K-2bits of the synchronisation pattern even if it is assumed that everyvariable bit of the string provides a match.
 11. A telecommunicationsystem comprising:first and second devices having communication meansfor providing time-division two-way communication between said devicesover a radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the first and second devices is completed before transmission of thenext of said alternating bursts by the other of the first and seconddevices is begun; means for producing bursts such that at least some arebursts of a particular format of digital data and comprise an L-bitsynchronisation pattern; means for asynchronously detecting saidsynchronisation pattern by the receiving device to enable it todetermine timing information about the bursts of said particular format;and means in the receiver to determine the synchronisation pattern to bepresent in the received data when a comparison operation between thereceived data and a stored copy of the synchronisation pattern resultsin no more than K bits of the received data failing the comparison,where K is zero or a positive integer; the synchronisation pattern beingadjacent a portion of the burst made up of fixed value bits, and thenumber of matches between the synchronisation pattern and any string ofL successive bits of the burst of said particular format composed onlyof at least a part of said portion of fixed value bits and an adjacentpart of the synchronisation pattern being less than L-K.
 12. A method oftelecommunication in which first and second devices communicate witheach other, comprising the steps of:performing time-division two-waycommunication between said devices over a radio channel by exchangingradio signals in alternating bursts carrying digital data, such thatduring the time-division two-way communication, transmission of one ofsaid alternating bursts from one of the first and second devices iscompleted before transmission of the next of said alternating bursts bythe other of the first and second devices is begun; producing burstssuch that at least bursts of a particular format of digital datacomprise an L-bit synchronisation pattern; asynchronously detecting saidsynchronisation pattern by a receiver to enable it to determine timinginformation about bursts of said particular format; determining thereceiver that the synchronisation pattern is present in the receiveddata when a comparison operation between the received data and a storedcopy of the synchronisation pattern results in no more than K bits ofthe received data failing the comparison, where K is zero or a positiveinteger; and arranging the burst of said particular format such that thesynchronisation pattern is adjacent a portion of the burst of saidparticular format made up of fixed value bits, and the number matchesbetween the synchronisation pattern and any string of L successive bitsof the burst of said particular format composed only of at least a partof the said portion of fixed value bits and an adjacent part of thesynchronisation pattern being less than L-K.
 13. A system according toclaim 11, wherein said burst producing means produces a presetsynchronisation pattern from among a predetermined plurality of L-bitsynchronisation patterns, and the number of matches between anyone ofsaid plurality of synchronisation patterns and any string of Lsuccessive bits of the burst composed only of any other of saidplurality of synchronisation patterns or at least a part of the saidportion of fixed value bits and an adjacent part of any other saidsynchronisation pattern is less than L-K.
 14. A system according toclaim 13 wherein, for at least some of the said plurality ofsynchronisation patterns, the said number of matches does not exceedL-K-8.
 15. A system according to claim 13, wherein, for all of the saidplurality of synchronisation patterns, the said number of matches doesnot exceed L-K-7.
 16. A system according to any one of claims 5, 7, 8,10, 11, and 13 to 15 in which K is not zero.
 17. A system according toclaim 16 in which K is two.
 18. A telecommunication systemcomprising:first and second devices having communication means forproviding time-division two-way communication between said devices overa radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the first and second devices is completed before transmission of thenext of said alternating bursts by the other of the first and seconddevices is begun; means for producing bursts such that at least some arebursts of a particular format of digital data and comprise an L-bitsynchronisation pattern; means for asynchronously detecting saidsynchronisation pattern by a receiver to enable it to determine timinginformation about the bursts of said particular format; thesynchronisation pattern having a peak self-correlation side lobe valueof not more than +2, for any amount of offset, where theself-correlation side lobe value at an amount of offset is defined asthe number of matches between bits of the pattern and itself offset bythe amount, minus the number of mismatches between the bits of thepattern and itself at the same amount of offset.
 19. A method oftelecommunication in which first and second devices communicate witheach other, comprising the steps of:performing time-division two-waycommunication between said devices over a radio channel by exchangingradio signals in alternating bursts carrying digital data, such thatduring the time-division two-way communication, transmission of one ofsaid alternating bursts from one of the first and second devices iscompleted before transmission of the next of said alternating bursts bythe other of the first and second devices is begun; producing burstssuch that at least bursts of a particular format of digital datacomprise an L-bit synchronisation pattern; asynchronously detecting saidsynchronisation pattern by a receiver to enable it to determine timinginformation about the bursts of said particular format; and arrangingthe synchronisation pattern to have a peak self-correlation side lobevalue of not more than +2, for any amount of offset, where theself-correlation side lobe value at an amount of offset is defined asthe number of matches between bits of the pattern and itself offset bythe amount, minus the number of mismatches between the bits of thepattern and itself at the same amount of offset.
 20. A telecommunicationsystem comprising:first and second devices having communication meansfor providing time-division two-way communication between said devicesover a radio channel by exchanging radio signals in alternating burstscarrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the first and second devices is completed before transmission of thenext of said alternating bursts by the other of the first and seconddevices is begun; means for producing bursts such that at least some arebursts of a particular format of digital data and comprise a 24-bitsynchronisation pattern which, when given in hexadecimal format, is oneof: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728;means for asynchronously detecting said synchronisation pattern by areceiver to enable it to determine timing information about the burstsof said particular format.
 21. A method of telecommunication in whichfirst and second devices communicate with each other, comprising thesteps of:performing time-division two-way communication between saiddevices over a radio channel by exchanging radio signals in alternatingbursts carrying digital data, such that during the time-division two-waycommunication, transmission of one of said alternating bursts from oneof the first and second devices is completed before transmission of thenext of said alternating bursts by the other of the first and seconddevices is begun; producing bursts such that at least bursts of aparticular format of digital data and comprise a 24-bit synchronisationpattern when, given in hexadecimal format, is one of: BE4E50; 41B1AF;EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728; asynchronouslydetecting said synchronisation pattern by a receiving device to enableit to determine timing information about the bursts of said particularformat.
 22. A method according to claim 2, wherein said time-divisiontwo-way communication may be performed by any one of a plurality ofdevices of the first type, and the plurality of devices of the firsttype do not respond to reception of the first synchronisation pattern orpatterns, whereby each of the plurality of devices of the first typedoes not respond to transmissions by any other device of the first type.23. A method according to claim 2 or claim 22, wherein the step ofproviding said bursts of a particular format includes a said device ofone type transmitting a predetermined synchronisation pattern whileattempting to initiating communication by said radio signals with adevice of the other type, and subsequently transmitting a differentpredetermined synchronisation pattern after said communication has beeninitiated.
 24. A method according to claim 6, wherein the arrangement ofbits in each said burst of said particular format is such that in anyconsecutive string of L bits of data in the burst of said particularformat, there are no more than L-K-6 bits of variable data.
 25. A methodaccording to claim 9, wherein any said string of L successive bits ofthe said repeating pattern matches no more than L-K-2 bits of thesynchronisation pattern even if it is assumed that every variable bit ofthe string provides a match.
 26. A method according to claim 12, whereinsaid step of producing bursts of a particular format comprises producinga preset synchronisation pattern from among a predefined plurality ofL-bit synchronisation patterns, and the number of matches between any ofsaid plurality of synchronisation patterns and any string of Lsuccessive bits of the particular burst composed only of any other saidsynchronisation pattern or at least a part of the said portion of fixedvalue bits and an adjacent part of any other said synchronisationpattern is less than L-K.
 27. A method according to claim 26, wherein,for at least some of the said plurality of synchronisation patterns, thesaid number of matches does not exceed L-K-8.
 28. A method according toclaim 26, wherein, for all of said plurality of synchronisationpatterns, the said number of matches does not exceed L-K-7.
 29. A methodaccording to any one of claims 6, 9, 12, 24, 25, 26, 27 and 28, whereinK is not zero.
 30. A method according to any one of claims 6, 9, 12, 24,25, 26, 27 and 28, wherein K is two.
 31. A telecommunication device forestablishing time-division two-way communication with another deviceover a radio channel by exchanging radio signals in alternating burstscarrying digital data such that, during the time-division two-waycommunication, reception of a said burst from said other device iscompleted before transmission of the next burst by saidtelecommunication device is begun;said telecommunication devicecomprising: detecting means for asynchronously detecting a firstsynchronisation pattern or one of a group of first synchronisationpatterns in a received burst to enable the telecommunication device todetermine timing information about said received burst; and means forproducing said bursts such that at least some of the bursts contain asecond synchronisation pattern or one of a group of secondsynchronisation patterns; said first synchronisation pattern or patternsbeing different from said second synchronisation pattern or patterns andsaid detecting means not responding to said second synchronisationpattern or patterns.
 32. A telecommunication device for establishingtime-division two-way communication with another device over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data such that, during the time-division two-way communication,reception of a said burst from said other device is completed beforetransmission of the next burst by said telecommunication device isbegun;said telecommunication device comprising: means for producingbursts for transmission by the telecommunication device at least some ofwhich are bursts of a type containing bits of variable data and asynchronisation pattern of L bits of data, the arrangement of bits inbursts of said type being such that any consecutive string of L bits ofdata in the burst contains less than L-K consecutive bits of variabledata, where K is a predetermined value which is zero or a positiveinteger.
 33. A telecommunication device for establishing time-divisiontwo-way communication with another device over a radio channel byexchanging radio signals in alternating bursts carrying digital datasuch that, during the time-division two-way communication, reception ofa said burst from said other device is completed before transmission ofthe next burst by said telecommunication device is begun;saidtelecommunication device comprising: detecting means for asynchronouslydetecting a synchronisation pattern of L bits of data in a receivedburst to enable the telecommunication device to determine timinginformation about the burst; and decoding means for decoding at leastsome received bursts according to a predetermined burst format; saiddetecting means determining that said synchronisation pattern is presentin received data when a comparison operation between the received dataand a stored copy of the synchronisation pattern results in no more thanK bits of the received data failing the comparison, where K is zero or apositive integer; and said predetermined format being such that a burstin said format contains bits of variable data and said synchronisationpattern, and in any consecutive string of L bits of data in a burst insaid format there are less than L-K consecutive bits of variable data.34. A telecommunication device for establishing time-division two-waycommunication with another device over a radio channel by exchangingradio signals in alternating bursts carrying digital data such that,during the time-division two-way communication, reception of a saidburst from said other device is completed before transmission of thenext burst by said telecommunication device is begun;saidtelecommunication device comprising: means for producing bursts fortransmission by the telecommunication device including said alternatingburst such that at least some of the bursts produced by said means forproducing bursts are bursts of a particular format of digital data suchthat a burst in said particular format comprises a first portion havinga repeating pattern of fixed value bits and variable bits and a secondportion comprising an L bit synchronisation pattern, the L bitsynchronisation pattern and the repeating pattern of fixed value bitsand variable bits being such that a string of L successive bits of therepeating pattern, starting at any position in a repeat of the pattern,matches less than L-K bits of the synchronisation pattern even if it isassumed that every variable bit in the string provides a match, where Kis a predetermined value which is zero or a positive integer.
 35. Atelecommunication device for establishing synchronous time-divisiontwo-way communication with another device over a radio channel byexchanging radio signals with said other device in alternating burstscarrying digital data such that, during the time-division two-waycommunication, reception of a said burst from said other device iscompleted before transmission of the next burst by saidtelecommunication device is begun;said telecommunication devicecomprising: detecting means for asynchronously detecting an L bitsynchronisation pattern in a received burst to determine timinginformation about the burst, said detecting means determining that saidsynchronisation pattern is present in received data when a comparisonoperation between the received data and a stored copy of thesynchronisation pattern results in a no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer; anddecoding means for decoding at least some received bursts according to aparticular format of digital data such that a burst in said particularformat comprises a first portion having a repeating pattern of fixedvalue bits and variable bits and a second portion comprising said L bitsynchronisation pattern; the L bit synchronisation pattern and therepeating pattern of fixed value bits and variable bits being such thata string of L successive bits of the repeating pattern, starting at anyposition in a repeat of said pattern, matches less than L-K bits of thesynchronisation pattern even if it is assumed that every variable bit inthe string provides a match.
 36. A telecommunication device forestablishing time-division two-way communication with another deviceover a radio channel by exchanging radio signals in alternating burstscarrying digital data such that, during the time-division two-waycommunication, reception of a said burst from said other device iscompleted before transmission of the next burst by saidtelecommunication device is begun;said telecommunication devicecomprising: means for producing bursts for transmission by thetelecommunication device at least some of which are bursts of aparticular format of digital data such that a burst in said particularformat comprises an L bit synchronisation pattern adjacent a portion ofthe burst made up of fixed value bits, the L bit synchronisation patternand said fixed value bits being such that the number of matches betweenthe synchronisation pattern and any string of L successive bits of aburst in said particular format which string is composed only of atleast a part of said portion of fixed value bits and an adjacent part ofthe synchronisation pattern is less than L-K, where K is a predeterminedvalue which is zero or a positive integer.
 37. A telecommunicationdevice for establishing time-division two-way communication with anotherdevice over a radio channel by exchanging radio signals in alternatingbursts carrying digital data such that, during the time-division two-waycommunication, reception of a said burst from said other device iscompleted before transmission of the next burst by saidtelecommunication device is begun;said telecommunication devicecomprising: detecting means for asynchronously detecting an L bitsynchronisation pattern in a received burst to determine timinginformation about the burst, said detecting means determining that saidsynchronisation pattern is present in received data when a comparisonoperation between the received data and a stored copy of thesynchronisation pattern results in a no more than K bits of the receiveddata failing the comparison, where K is zero or a positive integer; anddecoding means for decoding at least some received bursts according to aparticular format of digital data such that a burst in said particularformat comprises an L bit synchronisation pattern adjacent a portion ofthe burst made up of fixed value bits; the L bit synchronisation patternand said fixed value bits being such that the number of matches betweenthe synchronisation pattern and any string of L successive bits of aburst in said particular format which string is composed only of atleast a part of said portion of fixed value bits and an adjacent part ofthe synchronisation pattern is less than L-K.
 38. A telecommunicationdevice for establishing time-division two-way communication with anotherdevice over a radio channel by exchanging radio signals in alternatingbursts carrying digital data such that, during the time-division two-waycommunication, reception of a said burst from said other device iscompleted before transmission of the next burst by saidtelecommunication device is begun;said telecommunication devicecomprising: means for producing bursts for transmission by thetelecommunication device at least some of which are bursts of aparticular format of digital data such that a burst in said particularformat comprises an L bit synchronisation pattern having a peakself-correlation side lobe value of not more than +2 for any amount ofoffset, where the self-correlation side lobe value at an amount ofoffset is defined as a number of matches between bits of the pattern anditself offset by the amount, minus the number of mismatches between bitsof the pattern and itself at the same amount of offset.
 39. Atelecommunication device for establishing time-division two-waycommunication with another device over a radio channel by exchangingradio signals in alternating bursts carrying digital data such that,during the time-division two-way communication, reception of a saidburst from said other device is completed before transmission of thenext burst by said telecommunication device is begun;saidtelecommunication device comprising: means for asynchronously detectinga predetermined synchronisation pattern in a received burst to determinetiming information about the burst, the predetermined synchronisationpattern being such that it has a peak self-correlation side lobe valueof not more than +2 for any amount of offset, where the self-correlationside lobe value at an amount of offset is defined as a number of matchesbetween bits of the pattern and itself offset by the amount, minus thenumber of mismatches between bits of the pattern and itself at the sameamount of offset.
 40. A telecommunication device for establishingtime-division two-way communication with another device over a radiochannel by exchanging radio signals in alternating bursts carryingdigital data such that, during the time-division two-way communication,reception of a said burst from said other device is completed beforetransmission of the next burst by said telecommunication device isbegun;said telecommunication device comprising: means for producingbursts for transmission by the telecommunication device at least some ofwhich are in a particular format of digital data such that a burst insaid particular format comprises a 24-bit synchronisation patternselected from the group, defined in hexadecimal format: BE4E50; 41B1AF;EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728.
 41. Atelecommunication device for establishing time-division two-waycommunication with another device over a radio channel by exchangingradio signals in alternating bursts carrying digital data such that,during the time-division two-way communication, reception of a saidburst from said other device is completed before transmission of thenext burst by said telecommunication device is begun;saidtelecommunication device comprising: means for asynchronously detectingin a received burst a synchronisation pattern selected from the group,defined in hexadecimal format: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D;F58D82; A0D8D7; and 5F2728, to determine timing information about theburst.
 42. A system according to claim 13 wherein said first devicecomprises a said means for producing bursts of said particular formatsuch that a particular burst of said particular format from the firstdevice comprises a first one of said plurality of synchronisationpatterns, and said second device comprises a said means for producingbursts of said particular format such that a particular burst of saidparticular format from the second device comprises a second one of saidplurality of synchronisation patterns.
 43. A system according to claim13 wherein said burst producing means comprises means to variably selectdifferent said preset synchronisation patterns from among said pluralityof synchronisation patterns at different times.
 44. A method accordingto claim 26 wherein said preset synchronisation pattern comprises afirst synchronisation pattern or one of a first group of synchronisationpatterns when said step of producing bursts of said particular format iscarried out in said first device and said preset synchronisation patterncomprises a second synchronisation pattern or one of a second group ofsynchronisation patterns when said step of producing burst of saidparticular format is carried out in said second device.
 45. A methodaccording to claim 26 wherein said preset synchronisation pattern is adifferent one of said plurality of synchronisation patterns on differentoccasions when said step of producing particular bursts is carried out.46. A system according to claim 8 wherein said bursts of said particularformat are not said alternating bursts.
 47. A device according to claim34 wherein said bursts of said particular format are not saidalternating bursts.